HIGH  SCHOOL 
.KBORKTORY  MKNUKL 

°f 

PHYSICS 


INN  5  COMPANY 


Southern  Branch 
of  the 

University  of  California 

Los  Angeles 


FormL 


35 
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HIGH    SCHOOL 


LABORATORY   MANUAL 


PHYSICS 


BT 
DUDLEY  G.  HAYS       CHARLES  D.  LOWRY       AUSTIN  C.  RISHEL 

TEACHEKS  OF  PHYSIOS  IN  THE  CHICAGO  HIGH  SCHOOLS 


BOSTON,   U.S.A. 

GINN  &   COMPANY,   PUBLISHERS 
1895 


COPYRIGHT,  1893 
BY  GINN  &  COMPANY 


ALL  EIGHTS  RESERVED 


3.5 


PREFACE. 


~T~N  making  this  Manual  two  main  objects  have  been  kept  in 
view.  First,  the  teaching  of  Physics  by  the  Inductive 
Method,  that  is,  the  presenting  of  a  logically  arranged  course  of 
experimental  work  that  shall  covej  the  ground  of  Elementary 
Physics.  Second,  the  providing  of  sufficient  laboratory  work  to 
meet  the  entrance  requirements  of  any  college  in  the  country. 

The  authors  are  not  so  visionary  as  to  suppose  that  boys  and 
girls  can,  unaided,  rediscover  the  laws  of  Physics,  but  we  know 
that,  if  sufficiently  careful  directions  are  given  to  pupils  in  the 
performance  of  experiments,  and  definite  instruction  is  given  them 
as  to  the  manner  of  studying  results  obtained,  they  will  learn 
from  Nature  first-hand  many  of  her  great  laws.  And  these  will 
be  much  more  strongly  impressed  than  when  learned  from  a 
text-book  or  from  the  teacher's  experiments.  From  the  general- 
izations made  by  the  pupils,  deductions  can  be  made  and  then 
tested  for  their  validity,  thus  keeping  fhe  pupil  on  the  border- 
land between  inductions  and  deductions,  a  place  where  the  greatest 
mental  development  is  obtained.  This  plan  is  the  underlying 
idea  of  science  study,  and  the  acquiring  of  this  scientific  method 
is  of  far  more  importance  than  the  mere  gleaning  of  facts.  The 
facts  may  in  time  be  forgotten,  but  the  method  of  mind  opera- 
tion will  remain  as  part  of  the  character  developed. 

Careful  manipulation,  accurate  observation  of  phenomena,  and 
logical  deductions  or  generalizations  should  be  the  three  steps 
kept  in  mind. 

The  notes  written  by  the  pupils  should  be  neat,  terse,  and  in 
good  English,  and  should  be  logically  arranged  as  shown  in  the 
steps  mentioned  above.  Insist  on  these  three  elements  of  written 

work. 

iii 


IV  PREFACE. 

In  the  attainment  of  our  second  object,  the  requirements  of 
Harvard,  as  being  higher  and  more  definite  than  those  of  any 
other  institution,  have  been  our  standard,  and  we  acknowledge 
great  obligations  to  the  Harvard  Pamphlet. 

Equivalents  of  nearly  all  their  exercises  will  be  found  in 
this  Manual. 

With  regard  to  the  method  of  laboratory  work,  that  of  having 
duplicate  apparatus  so  that  a  whole  class  may  be  performing  the 
same  experiment  is  of  course  the  best.  Much  of  the  value  of 
the  work  is  in  the  discussion  t>f  the  results  in  class-room.  With 
this  in  view  the  apparatus  selected  is  of  the  simplest  possible 
kind  that  will  secure  good  results,  and  most  of  it  may  be  dupli- 
cated at  small  cost. 

In  the  arrangement  of  topics,  Magnetism  has  been  put  first 
because  the  experiments  are  easy,  instructive,  and  fascinating, 
thus  giving  a  very  desirable  introduction  to  the  laboratory  work. 
It  also  gives  the  teacher  time  to  prepare  his  laboratory  for  the 
more  difficult  work  which  comes  later.  The  other  topics  are 
arranged  in  about  the  usual  order. 

It  is  of  course  to  be  borne  in  mind  that  this  is  in  no  sense 
a  text-book,  nor  intended  to  supplant  one.  It  is  simply  a  lab- 
oratory manual  and  may  be  used  with  any  text. 


LABORATORY   MANUAL. 


MAGNETISM. 

The  beginning  work  in  the  laboratory,  covering  the  subject  of 
Magnetism,  can  be  performed  by  the  pupils  working  in  unison 
under  the  oral  directions  of  the  instructor,  who  can  emphasize  his 
directions  in  such  a  way  as  to  leave  no  doubt  as  to  the  way  to  pro- 
ceed. The  pupils  can  thus  be  given  a  start  in  an  earnest  manner, 
and  if  the  instructor  is  full  of  life,  his  pupils  will  soon  gain  a 
mental  momentum  that  will  count.  Enthusiastic  work  coupled 
with  neatness  and  accuracy  should  be  xthe  first  aim.  Unison  work 
calls  for  but  little  apparatus  here.  Let  each  pupil  bring  two 
knitting  needles,  and  the  jeweller  will  gladly  give  you  a  pocket  full 
of  old  watch  springs  which  are  excellent  material  for  magnets. 
The  magnetoscopes  should  be  prepared  by  the  instructor  before- 
hand. Fasten  a  silk  fibre  to  the  bottom  of  a  cork  of  a  small  bottle, 
such  as  a  Florence  flask,  and  to  the  lower  end  of  the  fibre  fasten  a 
short  piece  of  magnetized  watch  spring.  Clip  the  magnet  with 
tinners'  shears  till  it  is  balanced  and  is  short  enough  to  swing 
freely  in  the  bottle.  One  for  each  couple  of  pupils  will  be  enough. 
They  can  be  kept  throughout  the  year  and  will  be  of  frequent  use. 

Bough  notes  can  be  taken  in  the  laboratory  and  more  elaborate 
writing  can  be  prepared  from  them  at  home  for  the  first  few  days. 
Nothing  but  neatness  and  accuracy  of  statements,  based  upon  facts 
observed,  should  be  accepted.  A  right  beginning  is  half  the  battle. 

EXERCISE  1.  —  Mix  some  tacks,  some  bits  of  brass,  copper,  soft 
iron  wire,  and  some  iron  filings  on  a  piece  of  paper.  Stir  the 
mixture  with  one  end  of  a  knitting  needle,  and  note  whether  any 
of  the  objects  cling  to  the  needle.  Now  rub  the  needle  from  end 
to  end  several  times,  always  in  the  same  direction,  on  a  piece  of 
natural  magnet  or  one  end  of  a  bar  magnet  —  an  electro-magnet 
arranged  by  the  teacher  can  be  used  —  and  again  stir  the  mixture. 


4  LABORATORY  MANUAL. 

Do  any  of  the  objects  cling  to  the  needle  ?  What  ones  ?  Try  to 
pick  up  a  match ;  a  pin ;  a  sewing  needle ;  a  steel  pen.  What 
kinds  of  material  cling  to  the  needle  ?  How  does  the  needle  differ 
now  from  its  condition  when  first  tried  ? 

This  attraction  it  manifests  is  due  to  the  force  called  magnetism. 
The  needle  is  now  a  magnet.  What  substances  are  attracted  and 
held  by  a  magnet  when  touched  by  it  ? 

EXERCISE  2.  —  You  have  seen  that  when  certain  substances 
have  been  touched  by  a  magnet  they  will  cling  to  it.  Ascertain 
whether  it  is  necessary  to  touch  these  substances  in  order  to  show 
the  attraction  of  a  magnet  for  them.  Cut  out  a  piece  of  paper 
three  inches  long  and  two  inches  wide.  Fold  the  ends  together 
and  crease  the  paper  at  the  bend.  Pin  the  two  ends  together  with 
a  soft  iron  nail  parallel  to  the  ends  of  the  strip.  Now  pass  a 
thread  through  the  loop  above  the  nail  and  hang  it  to  any  support, 
as,  for  example,  over  a  ruler  projecting  from  the  edge  of  the  table, 
so  that  the  nail  may  swing  freely.  Hold  a  paper  before  one  end  of 
a  magnet,  and  bring  that  end  of  the  magnet  near  one  end  of  the 
nail.  Does  the  nail  turn  toward  the  magnet  ?  Bring  it  near  the 
other  end  of  the  nail,  being  careful  to  keep  the  paper  between 
magnet  and  nail.  Does  the  magnet  attract  each  end  of  the  nail 
even  though  it  does  not  come  in  contact  with  it  ?  How  does  this 
test  of  the  effect  of  a  magnet  on  soft  iron  or  steel  differ  from  that 
in  Exercise  1  ?  Is  contact  necessary  for  a  magnet  to  attract  soft 
iron? 

EXERCISE  3.  —  Pour  a  row  of  iron  filings  on  a  piece  of  paper  a 
little  longer  than  the  knitting  needle  magnet,  so  that  you  can  lay 
the  magnet  on  the  paper  and  have  it  rest  its  entire  length  in  filings. 
Eoll  the  magnet  back  and  forth  in  the  filings  and  then  pick  it  up 
carefully.  Note  whether  the  filings  cling  uniformly  along  the 
entire  length  of  the  magnet.  Strip  the  filings  off  and  repeat. 
Where  do  the  filings  cling?  Try  to  pick  up  tacks  at  different 
points  along  the  magnet.  Stand  a  book  on  one  end.  Lay  the 
magnet  on  the  book  so  that  one  end  —  nearly  half  —  of  the  magnet 
projects  beyond  the  edge  of  the  book.  Now  find  how  many  tacks 
will  hang  in  line  at  the  end  of  the  magnet.  Try  it  for  different 
places  along  the  magnet,  going  from  the  end  toward  the  center, 
the  places  being  about  a  half  inch  apart.  Kecord  the  results  for 
each  trial. 


6  LABORATORY  MANUAL. 

The  places  on  a  magnet  where  the  filings  cling  in  bunches,  or 
where  the  greatest  number  of  tacks  cling,  are  called  the  poles  of 
a  magnet. 

EXERCISE  4.  —  Get  a  heavy  sewing  needle  —  what  is  better,  a 
short  piece  of  Avatch  spring  one  inch  long  —  and  dip  it  into  filings 
to  be  sure  it  is  not  magnetized.  Now  lay  it  011  a  thin  cork  and  float 
it  in  a  dish  of  water.  If  the  dish  is  small,  it  is  well  to  oil  the  cork, 
which  will  then  remain  in  the  centre  of  the  dish.  When  it  has 
come  to  rest,  with  your  pencil  carefully  turn  it  around  so  that  it 
will  be  at  right  angles  to  the  position  it  had  when  at  rest,  and  note 
whether  it  will  stay  in  about  the  position  in  which  you  put  it. 
Now  remove  the  needle  and  stroke  it  on  one  end  of  a  magnet, 
always  from  end  to  end  in  the  same  direction.  Then  place  it  back 
on  the  cork  and  repeat  the  above  work.  What  follows  ?  Try 
other  needles  so  as  to  be  sure  of  the  results.  Does  the  same  pole 
always  seek  the  same  direction  ?  If  one  end  always  seeks  the  west, 
the  pole  at  that  end  would  be  called  the  west-seeking  pole.  Give 
the  correct  names  to  the  two  poles  of  the  magnets  you  have  tried. 

EXERCISE  5.  —  Cork  a  small  clear  glass  bottle  with  a  cork  having 
a  hole  in  it,  through  which  passes  a  small  glass  tube,  having  a  hook 
at  the  lower  end  to  which  is  tied  a  silk  fibre,  on  the  lower  end  of 
which  is  fastened  a  piece  of  watch  spring  which  has  been  magnet- 
ized, and  which  should  hang  horizontally  in  the  bottle  near  the 
bottom.  The  tube  can  be  moved  up  or  down  in  the  cork  to  adjust 
the  height.  The  silk  fibre  allows  the  magnet  to  move  freely.  This 
little  device  is  called  a  magnetoscope.  Bring  the  point  of  a  soft 
iron  nail  near  the  north-seeking  pole  of  the  magnetoscope.  What 
follows  ?  Do  the  same  with  the  head  of  the  nail.  What  follows  ? 
Now  get  a  knitting  needle  magnet  or  a  sewing  needle  magnet  and 
try  both  ends  of  it  the  same  way.  What  follows  ?  How  does  the 
action  when  the  nail  was  used  differ  from  that  when  the  magnetized 
needle  was  used  ?  You  thus  see  that  the  presence  of  magnetism  in 
a  body  may  be  detected  by  use  of  the  magnetoscope. 

EXERCISE  6.  —  In  the  last  exercise  we  saw  that  a  magnetoscope 
could  be  used  to  detect  the  presence  of  magnetism.  You  saw  that 
the  effect  of  one  end  of  the  magnet  on  the  north-seeking  pole  of  the 
magnetoscope  was  the  same  as  the  effect  of  the  point  or  the  head 
of  the  nail,  but  that  the  other  end  of  the  magnet  affected  the  north- 


8  LABORATORY   MANUAL. 

seeking  pole  of  the  magnetoscope  in  quite  a  different  manner,  — 
that,  instead  of  the  north-seeking  pole  of  the  magnetoscope  being 
attracted  in  all  cases,  there  was  one  case  in  which  it  was  repelled. 
This  repellent  effect  is  the  true  gnide  to  follow  in  determining 
whether  or  not  a  body  is  magnetized.  Magnetize  a  piece  of  watch 
spring  and  float  it  on  a  cork  and  thus  ascertain  its  north-seeking 
pole.  Stick  this  end  through  a  small  bit  of  paper  to  label  it. 
Proceed  carefully  and  make  the  following  tests,  bringing  one  pole 
of  the  magnet  slowly  toward  one  pole  of  the  magnetoscope,  and 
record  results  : 

Pole  of  magnet.  Pole  of  magnetoscope.  Effect. 
North-seeking.                        North-seeking.  ? 

North-seeking.  South-seeking.  ? 

South-seeking.  South-seeking.  ? 

South-seeking.  North-seeking.  ? 

By  examining  the  above,  discover  a  law  of  the  effect  of  magnetic 
poles  on  each  other.  State  it.  How  many  poles  has  a  magnet  ? 
What  can  you  say  in  regard  to  these  poles  ? 

EXERCISE  7.  —  Magnetize  a  knitting  needle,  and  by  means  of 
a  magnetoscope  determine  its  poles.  Put  a  paper  flag  on  the 
north-seeking  pole.  Break  or  cut  the  magnet  in  two  at  the 
middle,  and  roll  the  half  which  is  labeled  in  iron  filings.  Does 
it  seem  to  have  two  poles  ?  Test  this  piece  for  polarity  with 
the  magnetoscope.  What  do  you  find  ?  Now  break  this  piece 
at  the  middle,  and  repeat  the  above  tests.  Results  ?  You  might 
continue  this  dividing  process  with  the  same  results  until  you 
reached  the  smallest  physical  division  possible  which  would  give 
you  a  single  molecule.  What  would  be  the  condition  of  each  of 
the  molecules  thus  separated  from  a  magnet  ?  How  do  you  think 
the  molecules  are  arranged  in  a  magnet,  judging  from  what  you 
have  found  in  this  experiment  ? 

Lay  one  bar  magnet  on  another  so  that  like  poles  will  be 
together.  Have  them  project  from  some  support  above  the  table, 
—  a  book  will  do.  Now  bring  a  paper  on  which  are  tacks,  or 
filings,  up  against  the  projecting  ends.  Note  the  quantity  that 
clings  to  the  magnets.  Now  superpose  the  magnets  so  that  unlike 
poles  will  be  together,  and  repeat  the  above.  Eesults  ?  Explain. 
If  you  had  several  flat  magnets,  how  would  you  fasten  them 
together  so  as  to  make  one  strong  magnet  ? 


10  LABORATORY  MANUAL. 

EXERCISE  8.  —  Magnetize  a  short  piece  of  watch  spring  or  a 
sewing  needle  by  rubbing  it  from  one  end  to  the  other,  always 
in  the  same  way,  with  the  north-seeking  pole  of  the  magnet,  and 
then  test  the  end  of  the  newly  made  magnet  which  last  left  the 
old  magnet  for  polarity.  Try  several  and  record  results.  Kepeat 
by  using  the  south-seeking  pole  of  the  magnet.  Kesults  ?  Can 
you  discover  a  law  for  the  arrangement  of  poles  in  magnetization  ? 
State  it. 

EXERCISE  9.  —  Make  a  feeble  magnet  of  a  piece  of  watch 
spring  by  stroking  it  once  or  twice  with  one  end  of  a  magnet. 
Test  it  for  polarity  by  means  of  the  magnetoscope.  Place  it  on 
the  table,  and  approach  its  north-seeking  pole  with  the  north- 
seeking  pole  of  a  bar  magnet.  Make  several  trials.  Does  the 
north-seeking  pole  of  the  small  magnet  jump  towards  the  north- 
seeking  pole  of  the  bar  magnet  ?  If  it  does  so  several  times,  again 
test  it  for  polarity.  How  do  you  account  for  this  result  ?  From 
this  experiment  do  you  think  any  magnet  might  have  its  poles 
reversed  ?  Try  several  magnets  made  of  watch  springs,  and  see 
whether  or  not  you  can  change  the  polarity  at  your  pleasure. 
How  can  you  weaken  a  strong  magnet  ?  Some  magnets  have 
their  poles  stamped  or  marked  in  the  steel  when  they  are  pur- 
chased. Can  we  always  rely  on  these  marked  names  for  the 
poles  of  a  magnet  ? 

EXERCISE  10.  —  Select  a  piece  of  soft  iron  wire  or  soft  iron  rod 
of  small  diameter  (a  soft  iron  nail  will  do)  and  test  it  with  a  mag- 
netoscope to  be  sure  it  is  not  magnetized.  Hold  one  end  of  the 
piece  of  soft  iron  in  some  iron  filings.  Place  your  index  finger  over 
the  other  end  of  the  iron,  and  with  your  other  hand  bring  one  pole 
of  a  magnet  down  on  the  opposite  side  of  the  finger  from  the  iron, 
the  finger  thus  preventing  the  magnet  from  touching  the  soft  iron. 
Keeping  the  objects  in  the  position  indicated,  withdraw  the  soft 
iron  from  the  filings.  Do  filings  cling  to  the  soft  iron  ?  Does  it 
seem  to  be  a  magnet  ?  Test  it  for  polarity  by  means  of  a  mag- 
netoscope while  the  magnet  is  thus  near  it.  How  does  the  end 
of  the  soft  iron  farthest  away  from  the  magnet  compare  in  polarity 
with  the  pole  of  the  magnet  used  ?  Change  ends  of  the  magnet 
and  try  again.  Change  ends  of  the  soft  iron  and  repeat.  If  a 
piece  of  soft  iron  thus  used  manifests  magnetism,  it  is  called  an 
induced  magnet.  What  can  you  say  about  the  arrangement  of  the 


12  LABORATORY   MANUAL. 

poles  of  an  induced  magnet  in  regard  to  the  pole  of  the  magnet 
used  ?  If  you  should  lay  several  soft  iron  nails  down  end  to  end 
in  a  line,  leaving  a  small  space  between  the  ends,  and  were  to  hold 
the  north-seeking  pole  of  a  strong  magnet  near  one  end  of  the  line, 
what  would  all  of  the  ends  pointing  towards  the  magnet  be  ?  All 
pointing  away  from  the  magnet  would  be  what  kind  of  poles  ? 

EXERCISE  11.  —  Lay  two  rulers  on  the  table  about  4  in.  apart 
and  parallel  to  each  other.  Place  a  small  magnet  beween  them, 
and  cover  the  three  objects  with  a  piece  of  writing  paper.  Now 
with  the  thumb  and  finger  sift  some  fine  iron  filings  on  to  the  paper 
from  a  height  of  about  two  feet.  The  outline  of  the  magnet  can 
be  made  more  distinct,  perhaps,  by  carefully  tapping  tne  paper 
with  your  lead  pencil,  being  careful  not  to  move  the  paper.  The 
filings  should  show  distinct  lines  radiating  from  the  poles  of  the 
magnet.  These  lines  of  filings  indicate  the  lines  of  magnetic  force 
which  constitute  the  magnetic  field  of  the  magnet.  Make  a  careful 
sketch  of  this  field. 

Now  proceed  as  before  and  map  out  the  magnetic  fields  for 
a  magnet;  for  the  field  when  a  north-seeking  pole  is  opposite  a 
north-seeking  pole  and  about  1  in.  from  it;  for  the  field  when  a 
north-seeking  pole  is  opposite  a  south-seeking  pole  and  about  1  in. 
away  from  it. 

By  using  several  horseshoe  and  several  bar  magnets  so  as  to 
have  the  poles  arranged  around  a  central  point,  some  interesting 
figures  may  be  mapped  out  with  filings. 


14  LABORATORY   MANUAL. 


MEASURING. 

EXERCISE  12.  —  Measure  the  length  and  width  of  your  table- 
top,  and  record  the  measurements  in  m.  and  decimals  thereof,  as, 
^===s==^=^==^==^===^=^==^===,  L-  2.145  m.     Measure  the  height 
J  of  your  table,  taking  the  average 
at  several  points. 

Measure   the   length  of    this 
page,  reading  to  quarters  of  mm. 
F'g-  !•  and  record  in  cm.,  and  fractions 

thereof,  as,  27.95  cm.  To  measure  accurately,  the  measuring  rod 
must  be  so  placed  that  the  graduated  edge  will  be  in  contact  with  the 
surface  to  be  measured.  It  is  also  best  to  begin  at  some  division  a 
little  from  the  end  of  the  measuring  rod,  as  indicated  in  Fig.  1. 

EXERCISE  13.  —  Apparatus  :  Metric  rule,  exterior  and  interior 
calipers,  sphere,  rectangular  prism  and  cylinder. 

With  the  calipers  get  the  diameter  of  the  sphere,  and  by  placing 
the  calipers  on  the  metric  rule,  get  the  diameter  in  cm.,  measuring 
to  quarters  of  mm.  Lest  the  ball  be  not  a  perfect  sphere,  take 
the  average  of  several  diameters.  Find  the  vol.  in  ccm.,  using 
the  formula  on  p.  150. 

Measure  the  prism,  and  find  its  vol.  in  ccm. 

Measure  the  cylinder,  and  find  its  vol.  in  ccm.,  using  the  for- 
mula on  p.  150. 

EXERCISE  14.  —  Place  several  thicknesses  of  paper  in  the  micro- 
§meter  calipers,  and  close  the  calipers  upon  them,  using  very  gentle 
pressure.  You  will  probably  need  the  assistance  of  your  teacher 
in  reading  your  calipers.  From  the  thickness  of  all,  find  the  thick- 
ness of  a  single  sheet. 

Measure  the  thickness  of  the  pieces  of  wire  given  you. 

Measure  the  thickness  of  a  bit  of  window  glass,  a  sheet  of  mica, 
a  hair,  or  any  other  object  whose  thickness  you  care  to  know. 

EXERCISE  15.  —  Clean  a  small  beaker,  wipe  it  dry,  place  it  on 
an  accurate  balance,  and  counterbalance  it  accurately  by  pouring 
sand  into  the  other  scale  pan.  Now  draw  from  a  burette  exactly 
25  ccm.  of  distilled  water  into  the  beaker,  return  the  beaker  to  the 
balance,  and  find  the  weight  of  the  water  in  g.  What  does  1  ccm. 
of  water  weigh  ?  In  case  no  burette  is  at  hand,  use  a  graduated 
vessel,  but  take  a  larger  quantity  of  water,  say  50  to  75  ccm. 


16  LABORATORY   MANUAL. 


'PROPERTIES  OF  MATTER. 

EXERCISE  16.  —  Select  a  two-hole  rubber  stopper  which  just  fits 
a  bottle.  (If  no  rubber  stoppers  are  at  hand,  an  ordinary  cork  will 
do,  if  it  be  first  wet,  or,  better,  boiled  in  paraffine,  to  make  it  air- 
tight.) 

Through  one  of  the  holes  pass  the  neck  of  a  small  funnel. 
Cover  the  other  hole  with  a  finger,  and  pour  water  into  the  fun- 
nel, pouring  quite  rapidly  at  first.  Does  it  run  into  the  bottle  ? 
Kemove  the  finger  an  instant.  What  results?  Why?  What 
does  this  experiment  show  concerning  water  and  air? 

EXERCISE  17.  —  Fill  a  test  tube  half  full  of  water,  then,  inclin- 
ing the  tube  a  little,  slowly  pour  alcohol  into  it,  letting  it  run  down 
the  side  of  the  tube,  being  very  careful  not  to  shake  the  tube. 
When  full,  cover  with  the  thumb  and  shake  vigorously.  Notice 
any  change  of  volume. 

Fill  a  test  tube  with  water  to  within  2  cm.  of  the  top.  Now 
drop  in  about  2  or  3  ccm.  of  finely  powdered  sugar,  a  little  at  a  time, 
and  notice  the  volume  when  all  the  sugar  is  dissolved.  Is  the 
volume  of  the  mixture  equal  to  the  sum  of  the  volumes  of  the 
water  and  sugar  ?  The  same  experiment  may  be  tried  with  salt 
instead  of  sugar.  What  property  of  matter  have  these  experi- 
ments illustrated? 

EXERCISE  18.  —  Hold  a  piece  of  glass  tubing,  30  cm.  or  more 
in  length  and  5  or  6  mm.  in  diameter,  in  the  flame  of  a  Bunsen 
burner  so  as  to  heat  it  about  10  cm.  from  one  end.  Hold  the  tube 
with  both  hands,  and  turn  it  steadily  so  as  to  heat  it  equally  on 
all  sides.  Try  to  bend  it  from  time  to  time  to  see  if  it  softens. 
When  thoroughly  softened,  remove  from  the  flame  and  quickly 
stretch  it  out  into  a  fine  glass  thread.  When  cool,  break  in  the 
middle  ;  or,  if  you  pulled  it  apart,  break  off  a  few  cm.  of  the 
finest  end,  and  see  if  the  thread-like  part  is  still  a  tube.  To  do 
this,  insert  the  small  end  in  water  and  see  if  you  can  blow  air 
through  it.  Examine  it  after  withdrawing  it  from  the  water. 

What  property  of  glass  allows  you  to  draw  it  out  thus  ? 

EXKRCISE  19.  —  Place  a  calling  card  on  the  top  of  a  cork  in  a 
bottle.  Upon  the  card,  immediately  over  the  cork,  lay  a  penny. 
Now  try  to  snap  the  card  from  under  the  penny  so  the  penny  will 


18  LABORATORY  MANUAL. 

drop  upon  the  cork  and  remain  there.  A  littls  patience  will 
enable  you  to  do  it  readily. 

Explain  why  the  penny  remains  behind. 

Suspend  a  bag  of  sand  weighing  2  to  4  Ibs.  by  a  piece  of  twine 
about  50  cm.  long.  Fasten  a  hook  to  the  under  end  of  the  bag, 
and  from  this  suspend  about  50  cm.  of  the  same  kind  of  twine. 
Now  take  hold  of  the  lower  end  of  this  second  cord,  and  pull 
steadily  downward  till  the  cord  breaks.  Which  piece  of  cord 
broke  ?  Arrange  as  before,  and  this  time  break  the  cord  by  a 
sudden  jerk  downward.  Which  piece  of  cord  broke  ?  Why  ? 

EXERCISE  20.  —  Into  a  flask  which  contains  water  to  a  depth 
of  3  or  4  cm.  place  a  close-fitting  cork  containing  a  glass  tube 
with  its  upper  end  drawn  to  a  point,  and  the  lower  end  reaching 
about  2  cm.  into  the  water.  The  tube  must  be  air  tight  in  the  cork. 

Now  blow  into  the  flask  until  a  considerable  quantity  of  air 
has  bubbled  out  of  the  lower  end  of  the  tube  through  the  water. 
Remove  the  lips  from  the  tube.  What  results  ?  How  did  the 
compression  to  which  you  subjected  the  air  affect  its  density  ? 
How  did  it  affect  the  expansive  force,  or  tension  ?  Why  does  the 
water  stop  flowing  ? 

EXERCISE  21. — First  Part.  Cut  off  a  given  length  of  solid 
rubber  cord,  or  rubber  tubing,  say  40  or  50  cm.  Now  stretch  it 
a  little,  release,  and  measure  again  very  carefully.  Repeat  several 
times,  stretching  more  each  time,  always  measuring  after  stretch- 
ing, to  see  if  you  can  reach  a  point  where  the  rubber  does  not 
return  to  its  original  length.  A  substance  which  uniformly 
resumes  its  original  form  is  said  to  be  perfectly  elastic.  Do 
you  find  rubber  to  be  perfectly  elastic? 

Second  Part.  For  three  pupils.  Fasten  securely  one  end  of 
a  piece  of  spring  brass  wire  No.  28,  B.  &  S.  gauge,  3  or  4  m.  long 
to  a  screw  or  hook,  or  other  support  on  the  table-top,  letting  the 
wire  extend  over  another  table,  if  one  table  is  too  short.  To  the 
other  end  attach  a  30-lb.  Chatillon  spring-balance. 

Near  each  end  of  the  wire  fasten  a  drop  of  sealing-wax  to  serve 
as  a  marker.  Lay  a  metric  rule  lengthwise  under  each  end  of  the 
wire.  Apply  a  tension  of  1  Ib.  to  the  balance.  This  should  stretch 
the  wire  straight.  Now  adjust  each  ruler  under  the  wire  so  the  side 
of  the  drop  of  wax  is  exactly  over  one  of  the  divisions  of  the  ruler, 
and  do  not  allow  the  rulers  to  move  throughout  the  experiment. 


20 


LABORATORY  MANUAL. 


Increase  the  tension  2  Ibs.  (see  Note  2).  Bead  positions  of  both 
drops  of  wax  to  quarters  of  mm. ;  the  difference  between  their 
movements  is  the  amount  the  wire  has  been  elongated.  Reduce  the 
force  to  1  Ib.  and  notice  if  the  wire  regains  its  original  length. 

Make  several  trials  similar  to  the  above,  increasing  the  tension 
2  Ibs.  at  a  time  until  permanent  elongation  is  produced,  remember- 
ing that  after  each  trial  you  are  to  reduce  the  tension  to  1  Ib. 

Tabulate  your  results  as  follows  :  — 


Material  of  Wire,  


Gauge  No.,  ... 


Length, 


FOBCE  (F). 

MOVEMENT 
OF  IST  MARKER. 

MOVEMENT 
OP  2o  MARKER. 

ELONGATION  (E). 

Ibs. 

mm. 
ii 

mm. 

mm. 

(4 

it 

« 

M 

Compare  F  and  E,  and  see  if  there  is  any  law  connecting  the 
stretching  force  and  the  elongation. 

NOTE  1.  — Very  much  of  your  succeeding  work  will  be  to  derive  such  laws. 
In  doing  so,  there  are  two  general  ways  of  comparing  the  results  of  experi- 
ments :  (1)  If  the  quotients  (ratios)  of  corresponding  results  are  always  the 
same,  then  the  results  are  said  to  be  directly  proportional  to  each  other  j  for 
example,  if  in  your  experiment  E  -r  F  (or  F  -r  E),  always  gives  the  same  quo- 
tient, then  we  say  that  the  stretching  force  is  directly  proportional  to  the  elon- 
gation. (2)  If  the  products  of  corresponding  results  are  always  the  same,  then 
the  results  are  inversely  proportional  to  each  other ;  that  is,  if  F  x  E  gives 
always  the  same  product,  we  shall  say  the  stretching  force  is  inversely  propor- 
tional to  the  elongation.  In  most  cases  the  quotients  or  products  will  not  be 
exactly  equal,  owing  to  errors  in  work.  If  the  differences  make  only  a  small 
part  of  the  whole  results,  the  law  may  be  stated.  If  the  variations  are  consid- 
erable, state  what  you  think  the  law  to  be,  and  say  it  is  approximately  proved. 
(3)  Sometimes  one  set  of  results  is  to  be  compared  in  either  of  the  above  ways 
with  the  squares  or  square  roots  of  another  set  of  results,  and  occasionally  with 
the  cubes  or  cube  roots. 

NOTE  2.  — The  spring-balances  are  made  to  be  used  with  the  hook  hanging 
vertically  downward  ;  in  any  other  position  the  index  stands  back  of  the  zero 
mark.  In  this  case,  a  "balance  correction"  must  be  added  to  your  reading. 
To  find  the  balance  correction  for  any  position,  hook  a  more  delicate  balance  to 
the  draw-bar,  and  see  what  force  is  necessary  to  bring  the  index  to  the  zero 
mark ;  or,  a  cord  attached  to  the  hook  may  be  passed  over  a  smooth-running 
pulley.  To  the  cord  attach  a  very  light  scale-pan  on  which  to  lay  weights  suf- 
ficient to  move  the  index  to  the  zero  point.  The  weight  of  the  scale-pan  and 
weights  is  the  correction  sought.  In  all  cases  where  accuracy  is  sought,  the 
balance  correction  should  be  made. 


22 


LABORATORY  MANUAL. 


EXERCISE  22.  —  Support  two  of  the  prisms  described  on  p.  36 
upon  blocks  3  or  4  cm.  high,  so  their  upper  edges  shall  be  1  m. 
apart  ;  upon  these  lay  a  piece  of 
straight-grained  pine  or  basswood  3  ft. 
6  in.  long,  \  in.  thick,  and  1  in.  wide. 
Alongside  the  middle  of  the  rod  sup- 
port another  of  the  prisms  at  the 
same  height,  but  parallel  to  the  rod 
and  5  cm.  from  it.  Upon  this  prism, 
as  a  fulcrum,  rest  a  very  -light  rod 
32  cm.  long,  so  its  end  projects  2  cm. 
under  the  first  rod.  The  arrangement 
of  the  apparatus  should  be  as  shown 
in  Fig.  2. 


Fig.  2. 


Press  downward  upon  the  middle  of  the  rod  DE.  A  will  rise  ; 
and  since  A  C  is  five  times  the  length  of  CB,  A  will  rise  five  times  as 
far  as  B  is  depressed,  and  the  amount  of  rise  at  A  will  serve  to 
show  the  depression  at  B.  Consider  B  as  being  the  point  where 
the  rod  DE  touches  the  pointer  AB. 

Measure  the  height  of  A  above  the  table,  reading  to  halves  of  mm. 
Now  lay  a  weight  of  200  g.  upon  DE  at  B,  and  again  get  the  height 
of  A.  Kemove  the  weight  and  measure  again.  Now  use  weights 
of  400,  600,  800,  and  1000  g.,  reading  the  height  of  the  pointer  in 
every  case,  as  before,  and  stopping  at  any  time  permanent  bend- 
ing is  produced.  Great  care  should  be  exercised  in  handling  the 
weights  so  the  apparatus  is  not  jarred  and  the  position  of  the 
pointer  changed. 

In  case  other  than  flat  weights  are  used,  they  may  be  laid  on  a 
pan  suspended  from  the  under  side  of  DE  by  a  string  which  passes 
through  a  hole  in  the  table-top. 
Tabulate  the  results  as  follows  :  — 


First  case.  —  Rod  flat ;  supports  100  cm.  apart. 


LOAD. 

READINGS 
WITHOUT  LOAD. 

READINGS 
WITH  LOAD. 

RISE  OF 
POINTER 
IN  mm. 

DEPRES- 
SION 
OF  ROD 
IN  mm. 

DEPRES- 
SION 

PER  100   g. 

IN  mm. 

g. 

ii 

it 

24 


LABORATORY   MANUAL. 


Second  test.  —  Support  the  same  rod  on  its  narrow  side  with 
supports  100  cm.  apart,  and  load  from  500  to  2000  g.,  increasing 
500  at  a  time,  reading  as  before. 

Third  test.  —  Use  a  rod  of  same  material  and  grain,  but  1  cm. 
thick  and  1£  cm.  wide,  laid  flat,  with  the  supports  100  cm.  apart. 
Use  weights  of  100  to  500  g.,  increasing  100  g.  at  a  time. 

Fourth  test.  —  Use  the  same  rod  laid  flat  with  supports'  50  cm. 
apart,  with  forces  of  200  to  1,000  g.,  increasing  200  g.  at  a  time. 

A  study  of  your  tabulated  results  should  enable  you  to  answer 
these  questions :  — 

1.  What  is  the  relation  between  the  force  employed  and  the 
amount  of  bending  of  an  elastic  rod  ?    Kefer  to  Note  1,  p.  20. 

2.  What  is  the  relation  between  the  length  of  the  lod  and  the 
amount  of  flexure,  i.  e.,  depression  of  its  center? 

EXERCISE  23.  —  Arrange  apparatus  as  shown  in  Fig.  3.  A  is 
a  circular  board  12  in.  in  diameter  and  1  in.  thick.  B  is  a  rod  of 

ash  or  hickory,  \  in.  X  |  in. 
in  cross  section,  and  42  in. 
long,  clamped  with  its  cen- 
ter exactly  over  the  center 
of  A.  The  support  of  C 
should  be  adjustable  to 
heights  from  50  to  100  cm. 
above  A.  The  rod  is  firmly 
clamped  to  C.  It  is  best 
to  have  the  point  of  a  wire 
nail  project  downward  from 
the  center  of  A  into  a  hole 
bored  to  receive  it  in  the 
table-top,  so  as  to  prevent 
D  is  a  cardboard  with  an  arc  of  60°  gradu- 
in.  or  7  in.,  and  its 


Fig.  3. 


A  from  moving  about. 

ated  to  single  degrees  ;  the  radius  should  be 

center  should"  coincide  with  the  center  of  A. 

1.  Adjust  C  so  there  shall  be  a  space  of  100  cm.  between  the 
clamped  portions  of  the  rod,  and  apply  force  with  two  4-lb.  Chatillon 
balances  attached  to  the  cords  E  and  F,  pulling  in  opposite  direc- 
tions, with  equal  force,  and  keeping  the  cords  always  at  right 
angles  to  the  line  GH.  Use  forces  of  2,  4,  6  and  8  oz.  and  record 
for  each  force  the  number  of  degrees  of  torsion,  as  read  on  the 
card  Z>. 


26  LABORATORY  MANUAL. 

2.  Now  lower  C  so  that  but  50  cm.  of  B  are  exposed  to  torsion, 
and  use  forces  of  4,  8,  16  and  32  oz. 

3.  Substitute  a  similar  rod  of  f  in.  X  f  in.  sectional  area,  and 
with  a  length  of  100  cm.  use  forces  of  1  Ib.  to  4  Ibs. 

4.  Use  but  50  cm.  of  the  large  rod,  and  forces  of  2  Ibs.  to  8 
Ibs.,  using  larger  balances. 

If  in  any  cases  the  forces  suggested  are  not  suited  to  the 
strength  of  the  rods,  use  forces  that  are  suitable. 

Tabulate  the  results,  and  from  a  comparison  of  results  answer 
the  following  questions  :  — 

1.  What  is  the  relation  between  the  force  employed  and  the 
amount  of  torsion  ? 

2.  What  is  the  relation  between  the  length  of  the  rod  and  the 
amount  of  torsion  ? 

3.  How  nearly  do  your  results  agree  with  the  law,  "Torsion 
varies  inversely  as  the  fourth  power  of  the  diameter  of  the  rod  "  ? 

EXERCISE  24.  —  Apparatus :  about  1  m.  of  spring  brass  wire, 
No.  28  B.  &  S.  gauge,  and  a  30-lb.  ChatiUon  spring-balance.  The 
teacher  should  prepare  the  balance  for  use  by  slipping  an  empty 
thread  spool  over  the  hook  so  that  when  in  position  and  the  balances 
are  stretched,  the  axis  of  the  spool  is  at  right  angles  to  the  draw- 
bar of  the  balance.  Choose  a  spool  of  such  length  that  when 
properly  cut  away  at  the  ends  it  cannot  turn  on  the  hook. 

One  end  of  the  wire  should  be  wrapped  several  times  around  a 
cylindrical  support,  not  less  than  2  cm.  in  diameter.  A  gas-pipe, 
water-pipe  or  table  leg  will  answer.  After  wrapping  the  wire 
about  the  cylinder,  fasten  the  end  to  a  tack  or  screw  driven  near 
the  cylinder. 

The  other  end  of  the  wire  is  to  be  fastened  to  the  eye  in  the 
end  of  the  draw-bar  of  the  balances,  after  which  the  wire  should 
be  wrapped  several  times  around  the  spool  on  the  hook  of  the 
balance,  care  being  taken  all  the  time  not  to  twist  or  kink  the  wire. 
Be  careful  to  keep  your  hands  and  face  out  of  danger  in  case  the 
wire  recoils  after  breaking. 

The  apparatus  being  ready,  the  purpose  is  to  determine  the 
amount  of  force  necessary  to  break  the  wire.  Begin  with  a  small 
strain  on  the  wire,  holding  the  eyes  over  the  balance  so  that  the 
index  reading  of  the  balance  may  be  accurately  taken.  Increase 
the  force  steadily,  noting  all  the  time  the  reading  of  the  balances, 


28  LABORATORY  MANUAL. 

until  the  wire  breaks.  Eepeat  three  or  four  times,  using  a  different 
piece  of  wire  each  time,  and  thus  get  the  average  breaking  strength 
of  the  wire,  making  the  proper  balance  correction  as  noted  in 
Exercise  21. 

Now  test  a  wire  of  the  same  thickness  but  of  different  material, 
as  iron,  copper,  or  steel. 

Eeturning  to  the  first  case,  weigh  a  given  length  of  the  brass 
wire,  as  1  or  2  in.,  and  determine  what  length  of  it  would  just 
break  of  its  own  weight  if  suspended  by  one  end. 

EXERCISE  25.  —  Cut  a  lead  bullet  or  small  piece  of  sheet-lead 
in  two  with  a  sharp  knife,  then  press  the  freshly  cut  surfaces 
together  with  a  twisting  motion  until  they  cohere.  What  do  the 
facts  that  a  smooth  surface  and  pressure  are  necessary  indicate 
about  the  distance  through  which  cohesion  acts  ? 

In  same  way  unite  freshly  cut  surfaces  of  rubber,  paraffine  or  wax. 

Place  a  small  drop  of  mercury  on  the  table,  and  note  carefully 
its  shape.  Why  does  not  the  mercury  spread  out  over  the  table  ? 
Would  the  great  weight  of  mercury  hinder  or  assist  it  in  keeping 
a  spherical  form  ? 

Place  a  drop  of  water  on  a  clean  pine  board,  and  account  for 
the  shape  of  the  drop.  Now  place  drops  of  water  on  a  board 
that  is  covered  with  powdered  rosin.  Note  and  account  for  the 
difference  between  the  shape  of  these  drops  and  those  of  water 
on  a  clean  surface.  What  force  holds  the  powder  to  the  surface 
of  the  drop  ? 

EXERCISE  26.  —  Suspend  a  disc  of  tin  or  sheet-zinc  5  to  8  cm. 
in  diameter  by  means  of  three  threads  from  the  hook  on  the  under 
side  of  the  scale-pan  of  a  specific  gravity  balance.  Now  support 
the  balance  at  such  a  height  that  when  the  beam  is  horizontal,  the 
disc  rests  lightly  upon  the  surface  of  the  water  in  a  battery  jar  or 
tumbler.  Be  sure  there  are  no  air  bubbles  under  the  disc.  Care- 
fully lay  weights  on  the  other  scale-pan,  —  do  not  drop  them  in  — 
and  find  the  force  necessary  to  tear  the  disc  away  from  the  water. 
Look  now  at  the  under  side  of  the  disc  and  decide  which  is  the 
greater,  the  adhesion  of  the  water  to  the  disc,  or  the  cohesion 
of  the  water.  Was  the  force  overcome  that  of  cohesion  or  of 
adhesion  ? 

Wipe  the  disc  dry,  coat  its  under  surface  with  oil,  and  again 
find  the  force  necessary  to  tear  it  from  the  water. 


30 


LABORATORY  MANUAL. 


EXERCISE  27.  —  (To  the  teacher:  Sets  of  capillary  tubes,  cleaned 
for  use,  should  be  in  readiness  ;  tubes  1,  2  and  5  mm.  in  diameter 
and  20  cm.  long  are  suitable.  Liquids  for  cleaning  them  should  be 
in  readiness  so  the  pupils  lose  no  time.  A  rubber  bulb  with  rubber 
tube  attached  will  assist  in  drawing  the  liquids  through  the  tubes. 
To  clean  the  tubes,  wash  first  in  dilute  nitric  acid,  rinse  in  water, 
then  wash  in  dilute  caustic  potash  or  caustic  soda,  then  rinse  in 
water.) 

In  a  tumbler  or  beaker  place  distilled  water  about  6  or  8  cm. 
deep.  Lower  the  smallest  tube  to  the  bottom  of  the  tumbler,  so  as 
to  wet  the  tube  inside.  First  study  the  form  the  water  takes 
around  the  tube,  and  around  the  sides  of  the  tumbler.  What  form 
of  attraction  is  displayed  here  ?  Now  lift  the  tube  entirely  out  of 
the  water,  dry  the  end,  lower  it  very  slowly  toward  the  water  and 
watch  to  see  if  the  water  jumps  to  meet  the  tube.  Incline  the  tube 
a  little,  so  one  corner  approaches  the  water  first. 

Now  lower  the  tube  about  2  cm.  into  the  water,  holding  it 
vertically,  and  notice  at  what  height  the  water  stands  in  the  tube. 
Lower  still  more,  and  see  if  the  water  in  the  tube  remains  at  the 
same  height  above  the  water  in  the  tumbler.  Now  lift  the  tube 
entirely  out  of  the  water,  and  notice  if  any  water  remains  in  the 
tube,  and  if  so,  to  what  height.  Arrange  a  table  similar  to  the 
following,  and  with  the  other  tubes  and  liquids  determine  the 
facts : — 


DIAMETER  OF 
TUBE. 

HEIGHT 
IN  COLD  WATER. 

HEIGHT 
IN  HOT  WATER. 

HEIGHT 
IN  ALCOHOL. 

1  mm  
2  mm. 

6  mm  

When  the  large  tube  is  used,  what  is  the  shape  of  the  upper 
surface  of  the  water  in  the  tube  ? 

Pour  clean  mercury  into  a  test  tube  to  the  depth  of  about  5  cm., 
dry  the  tubes  and  place  them  one  at  a  time  into  the  mercury, 
holding  the  tubes  at  the  side  of  the  test  tube  so  you  can  see  if  the 
mercury  enters  the  tubes.  Record  your  observations.  Make  a 
sketch  showing  the  shape  of  the  mercury  surface  within  and 
around  the  largest  tube. 


32  LABORATORY  MANUAL. 

Take  two  plates  of  glass  about  3X8  cm.  Slips  for  mounting 
microscopic  specimens  are  convenient.  Lay  the  one  on  top  of  the 
other  with  a  bit  of  wood  2  mm.  thick  separating  them  their  entire 
length  along  one  edge.  Slip  a  rubber  band  around  them  so  as  to 
hold  them  firmly  together,  and  lower  them  vertically  into  the 
water.  Sketch  them,  showing  the  height  to  which  the  water 
rises  between  the  plates. 

These  phenomena  are  said  to  be  due  to  capillary  attraction. 
Can  you  explain  them  by  means  of  forces  already  studied  ? 


34 


LABORATORY  MANUAL. 


PENDULUMS. 

EXERCISE  28.  —  Suitable  pendulums  are  easily  made  from  lead 
bullets  by  cutting  into  one  side  with  a  knife,  inserting  a  thread 
and  pinching  the  lead  tight  about  the  thread  with  a  pair  of  forceps. 
Iron  balls  with  holes  through  them  will  also  serve.  These  pen- 
dulum bobs  should  be  in  readiness  for  the  class.  The  exercise  can 
be  worked  by  a  class  in  unison  if  desired. 

Measure  a  pendulum  60  cm.  long.  In  taking  its  length, 
measure  from  the  point  of  support  to  the  centre  of  the  bob.  The 
pendulum  may  be  suspended  between  the  thumb  and  finger,  if  you 
are  careful  to  give  it  a  definite  support  so  it  does  not  change  its 
length  during  the  experiment.  It  may  also  be  held  in  a  pair  of 
forceps,  hung  over  the  back  of  a  knife-blade,  or  suspended  in  a 
variety  of  ways. 

Start  the  pendulum  vibrating  through  an  arc  of  not  over  10  or 
15  cm.,  and  count  the  number  of  single  vibrations  in  a  minute. 
Repeat  two  or  three  times  before  recording.  Now,  keeping  the 
pendulum  still  the  same  length,  let  it  swing  through  a  much  larger 
arc,  say  30  to  40  cm.  Repeat  and  record.  What  influence  has  the 
length  of  the  arc  upon  the  number  of  vibrations  made  in  a  given 
time? 

Substitute  a  cork  bob,  or  other  light  body  for  the  metal  bob, 
retain  same  length  as  before,  and  determine  whether  the  material 
of  which  the  pendulum  is  made  affects  its  rate  of  vibration. 

Arrange  a  table  as  below,  and,  using  the  metal  bob,  record  your 
results  after  two  or  three  trials  with  pendulums  of  each  length  : 


NO.  VlBB.   PER  MlN. 

PERIOD,  i.  E.  TIME  OF  1  VIBR. 

LENGTH. 

* 

25  cm. 

100     " 

50     " 

200     " 

By  studying  your  results   determine  the  law  connecting  the 
period  of  a  pendulum  with  its  length.     (Note  1,  p.  20.) 


36  LABORATORY  MAX  UAL. 


MECHANICS. 
^^X 

For  applying  force  in  mechanics,  the  Chatillon  4-lb.  spring- 
balance,  graduated  to  ounces,  is  recommended.  Where  weights 
have  to  be  provided,  bags  of  sand  are  both  convenient  and  cheap. 
The  bag  should  be  made  of  closely  woven  material,  such  as  ducking. 

Suitable  levers  may  be  made  from  lattice  sticks  13  in.  long,  1 
in.  wide  and  £  in.  thick.  Half  an  inch  from  each  end  a  line  should 
be  drawn  across  the  broad  side,  and  between  these,  11  other  lines 
1  in.  apart ;  the  stick  now  resembles  a  foot  rule.  On  the  other 
broad  side,  exactly  opposite  each  line,  a  small  notch  should  be  cut 
across  the  lever.  A  triangular  prism  1  in.  long  and  £  in.  high, 
when  lying  on  one  face,  will  serve  as  a  fulcrum.  Where  metric 
weights  are  not  at  hand  in  sufficient  numbers,  nickels  will  serve  as 
5  g.  weights,  and  10  g.,  15  g.,  and  20  g.  can  be  cut  from  sheet- 
lead. 

Sets  of  two  single,  and  two  double  pulleys  should  be  provided ; 
also  stout  linen  or  cotton  cord  for  same.  Fishline  is  good. 

It  is  recommended  that  Exercise  40  be  performed  before  the 
class,  they  making  their  own  observations  and  deriving  the  laws 
unaided  so  far  as  possible.  A  wire  2  or  3  mm.  in  diameter  should 
be  stretched  across  the  room  on  a  grade  of  1  to  16 ;  by  means  of  a 
turnbuckle  it  can  be  stretched  very  tight.  On  the  wire  is  a  smooth- 
running  pulley  carrying  an  iron  weight  of  2  Ibs.  so  arranged  as  to 
roll  down  the  wire  when  released.  The  weight  is  held  in  place  at 
the  upper  end  of  the  wire  by  means  of  an  electromagnet.  If  the 
friction  prevents  the  weight  from  moving  1  foot  the  first  second, 
and  corresponding  integral  numbers  of  feet  for  each  succeeding 
second,  the  inclination  of  the  wire  should  be  increased  till  these 
results  are  reached.  A  seconds  pendulum,  hung  by  a  copper  wire, 
is  so  arranged  that  a  very  thin  wire  pointer  on  the  under  side  of 
the  bob  cuts  through  a  ridge  of  mercury  (a  trough  3  mm.  wide 
heaped  full)  at  the  middle  of  its  arc,  thereby  closing  a  circuit 
which  rings  a  bell  or  clicks  a  telegraph  instrument  once  a  second. 
The  ringing  of  the  bell  breaks  the  circuit  of  the  electromagnet, 
thus  releasing  the  weight.  Above  the  wire,  and  parallel  to  it  is 
stretched  a  cord  to  which  are  pinned  bits  of  paper  1  foot  apart,  to 
mark  the  distance  the  weight  moves. 


38 


LABORATORY  MANUAL. 


EXERCISE  29.  —  For  three  pupils.  On  a  smooth,  square  board 
rule  off  a  square  12  in.  on  each  side.  Divide  this  square  into  36 
two-inch  squares,  and  at  the  intersections  of  all  the  lines  bore  -J  in. 
holes  not  quite  through  the  board.  Number  these  holes  from  1  to 
49.  Provide  3  iron  pegs  fitting  the  holes  pretty  closely.  (Wire 
nails  broken  off  will  answer.) 


Lay  three  smooth  marbles  of  equal  size  upon  the  table,  and 
upon  these  the  board,  marked  side  up.  Attach  a  cord  a  foot  long 
to  each  of  the  pegs,  and  insert  the  pegs  in  three  holes  in  any  one 
line.  Hook  a  spring-balance  into  the  end  of  each  cord,  and  apply 
forces  at  right  angles  to  the  line  on  which  the  pegs  stand,  so  that 
when  all  the  balances  are  stretched,  the  board  will  not  move.  Be 
sure  the  cord  does  not  bear  down  upon  the  board,  and  that  it  lies 
exactly  over  the  line  traced  on  the  board.  Hold  the  balance  firmly 
in  both  hands,  with  the  knuckles  resting  upon  the  table,  and  keep 
the  head  in  such  a  position  that  at  a  given  signal  all  can  read  the 
balances  instantly  without  moving  anything  but  the  eyes.  Re- 
member that  the  percentage  of  error  is  much  less  if  the  force 
applied  is  considerable  than  if  it  is  small. 


40  LABORATORY  MANUAL. 

Now,  having  produced  equilibrium  and  taken  the  readings,  make 
a  diagram  of  the  board  in  your  note-book,  number  the  holes,  and 
record  the  conditions  which  produced  equilibrium.  The  record  can 
be  briefly  stated  in  the  form  of  a  table,  thus  :  — 


HOLE  NUMBER. 

FORCE. 

DIRECTION  :  N.,  S.,  E.,  OR  W. 





Now  change  the  distance  between  the  pegs,  and  record  the  con- 
ditions which  produce  equilibrium.  Be  sure  that  in  some  of  the 
trials  the  pegs  are  at  unequal  distances  from  each  other. 

Keturn  now  to  the  original  case  of  equilibrium;  let  all  the 
forces  retain  the  original  direction  and  magnitude,  and  two  of  them 
the  original  points  of  application.  Seek  a  new  point  of  application 
for  the  third  force  which  shall  leave  the  system  in  equilibrium  as 
before.  Take  each  of  the  other  two  forces  as  the  roving  one,  and 
record  the  results  of  your  trial,  together  with  any  conclusions  you 
may  have  reached. 

Again,  returning  to  any  of  the  cases  of  equilibrium,  see  if  the 
equilibrium  continues  while  the  forces,  keeping  the  same  magnitude 
and  points  of  application,  and  still  remaining  parallel  to  each  other, 
are  turned  about  into  new  directions,  i.  e.,  are  no  longer  at  right 
angles  to  the  line  of  pegs. 

"Now,  by  referring  to  your  results,  answer  the  following  ques- 
tions :  — 

1.  What  is  the  direction  of  the  equilibrant  of  two  parallel 
forces  acting  in  the  same  direction  ? 

2.  What  is  the  magnitude  of  this  equilibrant  ? 

3.  What  relation  exists  between  the  magnitude  of   the  two 
forces  and  the  distances  of  their  points  of  application  from  the 
point  of  application  of  the  equilibrant  ? 

EXERCISE  30.  —  Lay  the  prism  on  the  table,  and  on  its  edge 
place  the  middle  notch  of  the  lever.  This  support  on  which  the 
lever  turns  is  called  the  fulcrum.  If  the  lever  does  not  balance, 
lay  a  lead  chip  on  it  at  such  point  as  to  produce  equilibrium.  Note 


42 


LABORATORY  MANUAL. 


that  this  lever  balances  when  neither  end  shows  a  tendency  to  rise 
after  being  depressed. 

On  the  right-hand  end  of  the  lever  place  5  g.  6  in.  from  fulcrum ; 
in  all  cases  place  center  of  weight  exactly  over  the  line  on  the  lever. 
Call  this  5  g.  the  weight  to  be  lifted,  and  find  what  power  must  be 
applied  six  in.  to  left  of  fulcrum  to  produce  equilibrium.  Desig- 
nate the  weight  and  power  by  W  and  P,  the  fulcrum  by  /.  The 
distance  from  W  to  /  is  called  the  weight  distance,  Wd ;  that  from 
P  to  /,  the  power  distance,  Pd.  Place  your  results  in  the  proper 
place  in  the  following  table,  and  by  use  of  the  lever  find  all  the 
missing  terms  in  the  table.  For  convenience,  keep  Wd  to  your 
right :  — 


P. 

W. 

WD. 

PD. 

1 

? 

6g. 

Gin. 

6  in. 

2 

5 

10" 

3  " 

?  " 

3 

? 

15  " 

4  " 

3  " 

4 

10 

V  " 

2  " 

4  " 

5 

10 

15  " 

?  " 

6  " 

Now  place  the  fulcrum  under  the  notch  4  in.  from  one  end,  and 
balance  the  lever  with  lead  chips ;  then  .find  the  missing  terms  in 
the  following  table  :  — 


P. 

W. 

WD. 

PD. 

1 

5 

10 

4 

? 

2 

10 

? 

3 

6 

3 

? 

20 

4 

8 

Study  these  tables  to  see  if  you  can  find  a  general  relation  be- 
tween P,  W,  Pd,  and  Wd. 

Moments  of  Force.  The  moment  of  a  force  is  the  tendency 
of  that  force  to  produce  rotation  about  a  fixed  axis,  and  is  com- 
puted by  taking  the  product  of  the  force  times  the  distance  of  its 
point  of  application  from  the  axis  of  rotation.  Since  P  and  W  tend 
to  produce  rotation  about  the  fulcrum  in  opposite  directions,  call 
that  distance  positive  whose  moment  tends  to  motion  in  a  direction 
with  the  hands  of  a  watch,  and  the  other  negative.  Now,  compute 
the  moments  in  all  the  preceding  cases.,  arrange  the  results  as 


44 


LABORATORY  MANUAL. 


indicated   below,  being   careful  of   algebraic   signs,  and  find  the 
algebraic  sum  of  the  moments  in  each  case  :  — 


Sum  of  moments. 


2.   Etc. 

Now  calculate  the  moments  in  the  following  case  :  With/  4  in. 
from  end,  PF  20  g.,  Wd  4  in.,  balance  by  applying  two  weights  of 
5  g.  and  10  g.  at  different  places  on  the  other  arm. 

EXEKCISE  31.  —  Arrange  two   levers  as  shown  in  the  figure, 
using  a  loop  of  thread  to  connect 

/c  —  D//  them.      Balance  the  lever  with 

~~  lead  chips. 

^JJ  AB  is  the  lever  under  consid- 
eration. Since  you  learned  in  the 
preceding  exercise  that  a  power 
applied  downward  at  C  exerts  an  equal  upward  force  at  D  or  A, 
you  may  in  each  of  the  succeeding  tests  treat  the  power  as  applied 
upward  at  A,  although  you  apply  it  over  C.  On  AS  place  a  weight 
of  20  g.  6  in.  from  B.  What  is  the  Wd?  Pd?  Find  what  power 
applied  at  C  will  produce  equilibrium.  Fill  out  the  following  table  :  — 


P. 

W. 

PD. 

WD. 

1 

5 

? 

12 

6 

2 

? 

15 

12 

4 

3 

? 

no\ 

1  15J 

12 

{4 

4 

10 

20 

10 

? 

EXERCISE  32.  —  Arrange  the  levers  as  shown  in  the  figure.     A 
pin  may  be  driven  through  the  half-inch  mark  at  A,  bent  into  a 

hook,  which   is   then   hooked  into 
^|  another   bent   pin  driven  into  the 

/>A    \<'  x/    table-top.    Balance  the  levers.    AB 

*  is   the   lever   under   consideration. 

Fig- «•  Where  is  its  fulcrum?     If  power 

be  applied  at  (7,  where  does  it  exert  itself  upon  AB  ?   Lay  a  weight 


4G 


LABORATORY  MANUAL. 


of  5  g.  on  the  half-inch  mark  at  B,  and  apply  power  at  C  to  pro- 
duce equilibrium.  What  is  the  length  of  the  power  arm  on  tiie 
lever  AB  ?  The  weight  arm  ?  Arrange  yo*ur  results  in  the  first 
line  of  the  table.  Find  the  other  missing  terms  :  — 


p. 

W. 

PD. 

WD. 

1 

? 

5 

? 

? 

2 

20 

10 

4 

?     . 

3 

? 

{    &} 

4 

r  6 

\io 

4 

20 

• 

8 

10 

Compute  the  moments. 

EXERCISE  33. — Take  a  wheel  and  axle  device;  measure  the 
diameter  of  both  the  wheel  and  the  axle  ;  call  them  D  and  d.  What 
is  the  ratio  of  Dtod?  What  is  the  ratio  of  their  circumferences  ? 
Hang  a  weight  of  3  to  5  Ibs.  on  the  cord  passing  around  the 
axle,  hook  the  spring-balance  into  the  cord  passing  around  the 
wheel,  and  raise  the  weight,  pulling  vertically  downward.  Read 
the  balances  to  quarter  ounces  while  the  weight  rises,  and  add  the 
balance  correction  for  this  position  (page  20,  Note  2).  Now  let  the 
weight  slowly  descend,  reading  and  making  correction  as  before. 
The  average  of  these  two  readings  is  the  power  necessary  to  support 
the  weight  on  the  axle.  Repeat  your  readings  several  times  before 
recording.  Call  the  power  and  weight  P  and  W.  What  is  the 
ratio  of  P  to  W?  Compare  this  ratio  with  that  of  the  diameters. 
Can  you  see  a  general  relation  between  P,  W,  D,  and  d  ? 

How  far  must  the  power  act  to  raise  weight  10  cm.  ?  Do  you 
find  it  necessary  to  measure  to  answer  this  question  ? 

In  this  exercise,  which  is  gained,  intensity  or  velocity  ?  Which 
was  lost  ? 

Apply  W  to  wheel  and  P  to  axle,  and  state  which  is  gained  and 
which  lost,  intensity  or  velocity.  Calculate  the  work  done  by  P 
in  raising  W  4  in.*  Compare  this  with  the  work  necessary  to 
raise  W  directly  upward  the  same  distance. 

EXERCISE  34.  —  Arrange  your  apparatus  as  in  Fig.  7  (a),  using 
a  weight  of  3  to  5  Ibs.  Hook  a  spring-balance  which  registers  to 

*  The  work  done  by  any  power  is  the  product  of  the  power  times  the  distance  it  has 
moved. 


48 


LABORATORY  MANUAL. 


ounces  into  the  cord,  and  raise  the  weight  slowly,  applying  the 
power  as  nearly  vertically  downward  as  you  can ;  note  on  the 
balances  the  power  used,  reading  to 
half  ounces.  To  this  reading  add  the 
balance  correction  for  this  position  of 
the  balance.  Now  allow  the  weight 
to  descend  slowly  and  note  the  power 
used  as  before.  Kepeat  several  times ; 
then  take  the  average  of  the  power 
used  in  raising  and  lowering  as 
the  force  necessary  to  support  the 
weight. 

1.  What  is  the  ratio  of  P  to  W? 

2.  How  far  must  P  act  to  raise  W  6  in.  ? 

3.  What  is  the  ratio  of  Pd  to  Wd  ? 

4.  In  this  arrangement,  how  many  sections  of  the  cord  support 
the  weight  ? 

Arrange  the  pulleys  as  shown  in  the  other  figures,  and  answer 
the  above  questions  for  each  arrangement.  When  done,  arrange 
the  results  of  the  four  trials  in  the  following  table  : 


P. 

W. 

Pd. 

Wd. 

SECTIONS  OF  CORD. 

1 

2 

3 

4 

Can  you,  by  observing  P,  W  and  the  number  of  sections  of  the 
cord,  discover  a  general  law  for  the  pulley  ? 

Compute  the  work  done  by  the  power  in  the  last  case  in  raising 
the  weight  10  in.,  and  compare  this  with  the  work  necessary  to 
raise  the  weight  10  in.  directly  upward. 

EXERCISE  35.  —  Support  a  smooth  board  about  80  cm.  long  and 
30  cm.  wide  firmly  on  the  table  with  one  end  raised  from  half  to 
two-thirds  the  length  of  the  board  above  the  table.  The  board  so 
arranged  is  called  an  inclined  plane. 

Measure  the  length  of  the  plane.  Measure  its  height.  This 
is  the  vertical  distance  from  the  under  edge  of  the  upper  end  of 


50  LABORATORY   MANUAL. 

the  board  to  the  table.  Call  these  L  and  //.  Find  the  ratio  of 
H  to  L. 

Place  a  weight  of  2  to  5  Ibs.  on  the  cart,  —  a  roller  skate  will 
do,  —  and  weigh  the  cart  and  its  load.  Call  this  weight  W.  Place 
the  loaded  cart  on  the  plane,  and  apply  power  with  the  spring- 
balances  so  as  to  draw  it  steadily  up  the  plane,  being  careful  to 
apply  the  power  in  a  direction  parallel  to  the  plane.  Take  very 
careful  readings  —  at  least  to  half  ounces  —  as  the  weight  ascends, 
making  the  proper  balance  correction.  Now  allow  the  cart  to  roll 
down  the  plane,  taking  readings  at  it  moves.  Repeat  several 
times ;  then  take  the  average  of  the  readings  while  ascending  and 
descending  as  the  power  necessary  to  support  the  cart  on  the  plane, 
irrespective  of  friction.  Call  this  P,  and  find  the  ratio  between 
P  and  W.  Compare  this  with  ratio  of  H  to  L. 

How  far  must  the  power  act  to  draw  W  the  full  length  of  the 
plane  ?  How  far  has  W  risen  vertically  in  the  meantime  ?  Calcu- 
late the  work  done  by  P  in  raising  J^  8  in.,  and  compare  with  the- 
work  necessary  to  raise  W  8  in.  directly  upward. 

EXERCISE  36.  —  Place  a  smooth  pine  board  about  1  m.  long 
upon  the  table.  Take  a  smooth  pine  block  about  2  X  6  X  lo  in.  At 
the  center  of  one  end  of  the  block  fasten  a  screw-eye  with  cord 
attached. 

Place  the  block  with  one  of  its  narrow  faces  upon  the  board, 
and  with  the  balances  move  it  slowly  and  steadily  along  the  board, 
with  its  fibers  parallel  to  those  of  the  board,  being  careful  to  keep 
the  cord  parallel  to  the  table-top.  Record  the  force  necessary  to 
start  the  block ;  now  find  the  force  necessary  to  keep  it  in  motion. 
As  the  force  necessary  to  move  the  block  may  vary  at  different 
parts  of  the  board,  a  place  should  be  sought  where  for  several 
inches  the  force  is  uniform,  and  all  readings  taken  there.  Repeat 
the  process  several  times,  take  careful  readings,  make  the  balance 
correction  for  this  position  and  call  the  average  of  these  corrected 
readings  the  force  necessary  to  overcome  friction.  What  is  the 
amount  of  pressure  of  the  block  upon  the  board  ?  The  ratio  of 
friction  to  pressure,  i.  e.,  friction  divided  by  pressure,  is  called  the 
coefficient  of  friction.  Find  the  coefficient  in  the  above  case. 

Lay  the  board  on  its  broad  side,  and  find  the  coefficient.  Now 
place  a  weight  of  2  or  3  Ibs.  upon  the  block,  and  find  the  coefficient. 

Lay  the  block  upon  two  round  lead-pencils  and  find  the  coeffi- 
cient of  rolling  friction. 


52  LABORATORY  MANUAL. 

EXERCISE  37.  —  Through  one  corner  of  a  piece  of  cardboard,  6 
or  8  in.  across,  prick  a  hole  with  a  pin  and  enlarge  the  hole  so  the 
card  will  revolve  freely  about  the  pin.  Hang  a  plumb-line  from 
the  pin,  and  suspend  both  the  card  and  plumb-line.  It  will  be  well 
to  drive  the  pin  horizontally  into  a  wooden  support.  When  both 
have  come  to  rest,  place  a  mark  very  carefully  back  of  the  plumb- 
line  just  where  the  line  leaves  the  card.  Remove  the  card,  and 
with  ruler  and  sharp  pencil  trace  on  the  card  the  line  of  direction 
of  the  pendulum. 

Suspend  as  before  from  any  other  position  and  trace  the  line  as 
before.  At  the  intersection  of  these  two  lines  stick  a  pin  through 
the  card,  bend  it  into  a  hook  and  suspend  the  hook  by  a  thread. 
If  the  card  hangs  horizontally,  the  center  of  gravity  lies  half  way 
between  the  two  surfaces  at  the  intersection  of  these  lines. 

Kemove  the  hook,  find  a  new  line  of  direction  and  see  if  it 
intersects  the  two  other  lines  at  the  point  where  the  hook  was. 

EXERCISE  38.  —  Influence  of  Weight  of  Lever.  Apparatus  : 
Lever  and  fulcrum  used  in  Exercise  30  ;  metric  weights  and  ruler. 

Find  the  center  of  gravity  of  the  lever  by  balancing  it  over 
a  knife  edge,  or  other  sharp  support.  Weigh  the  lever  to  tenths 
of  a  g.  Place  the  notched  face  of  the  lever  upward,  lay  a  weight 
of  20  g.  on  the  half-inch  mark  at  one  end,  and  carefully  adjust  the 
fulcrum  so  the  lever  will  balance  without  using  additional  weights. 
Be  sure  the  fulcrum  is  exactly  cross-wise  under  the  lever.  Now 
measure  very  carefully  the  distance  from  weight  to  fulcrum,  also 
from  center  of  gravity  of  lever  to  fulcrum.  Compute  the  moment 
on  the  weight  side.  Find,  by  applying  the  law  of  moments,  at 
what  distance  from  the  fulcrum  the  weight  of  the  lever,  if  applied 
at  a  single  point,  would  have  to  act  to  produce  the  equilibrium 
observed. 

Now  place  the  weight  on  the  second  mark,  balance,  measure, 
and  compute  as  before. 

Again,  increase  the  weight  to  30  or  40  g.,  and  proceed  as  before. 

Consider  now  whether  in  every  case  the  weight  of  the  lever  has 
acted  as  if  applied  at  the  same  point ;  and,  if  so,  what  is  that 
point  ? 

EXERCISE  39.  —  On  a  drawing  board  fastened  firmly  against  a 
wall,  suspend  two  spring-balances,  reading  to  ounces,  as  shown  in 
Fi.  8. 


54 


LABORATORY   MANUAL. 


On  the  cord  BD  hang  a  weight  of  2  to  5  Ibs.  With  thumb- 
tacks or  pins  fasten  a  sheet  of  paper  to  the  board,  with  the  middle 

of  the  paper  back  of  the  inter- 
section of  the  cords  ;  using  a  ruler 
and  sharp  lead-pencil,  trace  on  the 
paper  lines  parallel  to  each  of  the 
three  cords,  being  sure  the  lines 
intersect  at  a  common  point. 

Head  the  balances,  and  beside 
each  line  mark  the  reading  in 
ounces.  Now  remove  the  paper 
from  the  board,  decide  on  some 
scale  suited  to  the  force  used  and 
size  of  the  paper,  and  mark  off  on 
each  of  the  three  lines  a  distance 
corresponding  to  the  force  applied 
in  that  direction.  Consider  AB 
and  BC  as  sides  of  a  parallelogram  ; 
finish  the  parallelogram  and  draw 
its  diagonal  BE.  On  the  scale 
adopted,  how  great  a  force  does  the  diagonal  represent  ?  Compare 
this  with  BD.  Considering  AB  and  BC  as  components,  what  are 
BE  and  BD  ?  What  truth  does  this  experiment  seem  to  teach  ? 

EXERCISE  40.*  —  Let  the  weight  roll  down  the  wire,  and  notice 
how  far  it  moves  in  one  second.  Kepeat  several  times.  Then  find 
the  distance  the  weight  moves  in  two  seconds,  etc.  Tabulate  the 
results  : 

Length  of  plane  ................  ft.     Height  of  plane  ................  ft. 

Entire  distance  passed  over  in 
One  second    ................................  ft. 

Two  seconds  ................................  " 

Three     "       ................................  " 

Four      "       ................................  " 

Five       "       ................................  " 

By  a  comparison  of  the  above  results,  fill  out  the  following 
table:  — 

Distance  passed  over  in 
First  second  ...............................  ft. 

Second    "       ................................  « 

Third      "       ................................  " 

Fourth    "       ................................  " 

Fifth       "  .....  «« 


*  For  directions  as  to  apparatus,  see  p.  36. 


56  LABORATORY   MANUAL. 

Let  the  distance  passed  over  during  the  first  second  be  repre- 
sented by  d.  Now  at  the  right  of  both  the  above  tables,  write 
the  number  of  d's  passed  over  in  each  of  the  periods  of  time  given 
in  the  tables. 

Compare  the  number  of  d's  passed  over  in  any  number  of 
seconds  with  that  number  of  seconds,  and  state  a  law,  if  you  can. 
(Law  I.) 

In  the  second  table,  compare  the  number  of  d's  passed  over 
during  each  second  with  the  number  of  that  second,  and  state  a 
law,  if  you  can.  (Law  II.) 

The  velocity  at  the  beginning  of  the  first  second  was  0 ;  it 
increased  uniformly  throughout  the  second  ;  the  average  velocity 
for  the  second,  therefore,  equals  half  the  sum  of  the  velocities 
at  the  beginning  and  end  of  the  second.  But  the  space  passed 
over  during  the  first  second  equals  the  average  velocity  for  that 
second.  How  many  d's  represent  the  velocity  at  the  close  of  the 
first  second  ? 

Now,  observing  that  the  velocity  at  the  close  of  the  first  second 
is  also  the  velocity  at  the  beginning  of  the  second  second,  and  also 
that  the  number  of  d's  passed  over  during  the  second  second  is  the 
average  velocity  for  the  second,  find  the  velocity  at  the  end  of  the 
second  second.  Proceeding  in  the  same  way,  make  a  table  for  the 
velocity  at  the  end  of  each  second. 

The  velocity  gained  each  second  is  called  the  acceleration  or 
increment  of  velocity  for  that  second.  How  many  d's  represent 
the  increment  for  each  second  ?  State  a  law  for  the  velocity  at 
the  end  of  any  second.  (Law  III.) 

Let  S  represent  the  distance  passed  over  in  any  number  of 
seconds  by  a  body  free  to  move,  acted  on  by  a  constant  force;  s,  the 
distance  passed  over  in  any  single  second ;  t,  the  time,  and  d,  as 
before,  the  distance  passed  over  during  the  first  second.  State 
Laws  I,  II,  III  by  means  of  formulae  ;  e.  g.,  S=  ?  s==?  V=? 

The  force  of  gravity  is  the  constant  force  here  employed,  and 
these  laws  will  apply  to  freely  falling  bodies,  provided  you 
multiply  your  value  of  d  by  the  ratio  of  the  length  of  the  incline 
to  its  height. 


58 


LAB  OR  A  TOR  Y   MANUAL. 


HYDROSTATICS. 

EXERCISE  41.  — Put  a  wet  card  on  one  end  of  a  chimney  and 
push  it  down  into  a  glass  jar  filled  with  water. 
What  keeps  the  card  from  leaving  the  chimney  ? 
Now  carefully  pour  water  into  the  chimney  and  note 
how  the  height  of  the  water  in  the  chimney  compares 
with  the  level  of  the  water  outside  the  chimney  when 
the  card  is  pushed  off.  Make  several  trials,  and  vary 
the  depth  to  which  you  push  the  chimney.  From 
your  results,  to  what  is  the  force  which  holds  the 
card  against  the  chimney  equal  ? 

Tie  a  piece  of  rubber  tissue  over  one  end  of  a 
chimney  and  after  filling  it  with  water  cork  the  other 
end  with  a  cork  through  which  pass  three  glass  tubes 
as  shown  in  Fig.  9.  Be  careful  to  have  the  centers 
of  the  lower  ends  of  the  tubes  on  the  same  level. 

Note  the  heigth  of  the  water  in  the  three  tubes. 
Subject  the  liquid  to  pressure  by  pressing  gently  on 
the  rubber.  Note  the  relative  heights  to  which  the 
water  is  pushed. 

How  did  the  pressure  get  to  the  water  in  the 
tubes  ? 

In  what  directions  did  the  water  move  when 
entering  the  tubes  ? 

What  facts  does  this  exercise  show  in  regard  to  the 


Fig.  9. 


transmission  of  pressure  to  which  water  is  subjected  ? 

EXERCISE  42.  —  Use  a  small  brass  bucket  and 
plug.  You  will  notice  that  the  plug  exactly  fills  the 
cavity  of  the  bucket.  Now  hook  the  plug  to  the 
bottom  of  the  bucket  and  suspend  the  bucket  from 
one  scale-pan  of  a  balance,  then  put  weights,  or  shot, 
or  sand  into  the  other  scale-pan  of  the  balance  till 
equilibrium  is  restored.  Now  place  a  dish  of  water 
under  the  plug  and  elevate  the  dish  (or  lower  the 
upright  standard  of  the  balance)  till  the  plug  is 
entirely  submerged  when  the  arms  of  the  balance  are 
held  horizontal.  Release  your  hold.  Is  the  equi- 
librium disturbed?  Carefully  pour  water  into  the 
bucket  till  the  arms  of  the  balance  are  horizontal. 


Fig.  10. 

The  apparent 


60  LABORATORY  MANUAL. 

loss  of  weight  of  the  plug  when  it  is  immersed  in  the  water  is 
due  to  the  buoyant  force  of  the  water  which  it  displaced.  How 
does  th«  volume  of  the  water  displaced  by  the  plug  compare  with 
the  volume  poured  into  the  bucket  ?  Judging  from  this,  the 
buoyant  force  of  a  fluid  upon  a  body  submerged  in  it  is  equal  to 
the  weight  of  what  ? 

Did  the  plug  lose  any  of  its  weight  ?  Did  it  lose  weight  so  far 
as  the  balance  reading  was  concerned  ?  It  is  customary  to  speak 
of  a  body  as  losing  weight  when  it  is  immersed  in  a  liquid,  but 
this  is  a  peculiar  use  of  the  word  "lose." 

The  truth  you  have  learned  in  this  exercise  is  called  Archimedes' 
principle. 


62 


LABORATORY  MANUAL. 


SPECIFIC  GRAYITY. 

EXERCISE  43.  —  Measure  very  carefully  a  rectangular  block 
about  3  cm.  X  5  cm.  X  10  cm.  Compute  its  volume.  Weigh  it. 
What  is  the  weight,  in  g.,  of  1  com.  of  this  wood  ?  This  number 
is  called  the  density  of  the  wood.  Balance,  with  sand,  a  beaker, 
then  pour  into  it  from  a  graduate  as  many  ccm.  of  water  as  will 
equal  the  volume  of  the  wood.  Weigh.  Divide  the  weight  of  the 
wood  by  that  of  the  water.  The  quotient  is  called  the  specific 
gravity  of  the  wood.  Formulate  a  definition  for  specific  gravity. 

EXERCISE  44.  —  Weigh  an  irregular  piece  of  some  solid  denser 
than  water  and  not  soluble  in  it.  Tie  a  fine  thread  to  the  solid, 
attach  it  to  one  side  of  the  balance  and  let  it  be  immersed  in  water. 
Re-weigh.  By  Archimedes'  principle  determine  the  volume  of  the 
solid.  What  is  the  weight  of  the  same  volume  of  water  ?  What, 
then,  is  the  specific  gravity  of  the  solid  ? 

In  like  manner  find  the  specific  gravity  of  at  least  6  substances. 

Tabulate  as  below  where  W  equals  weight  in  air ;  W  equals 
weight  in  water ;  D  equals  difference  between  your  result  and 
that  given  in  the  table  of  specific  gravity  in  the  back  of  this  book. 
This  difference  is  not  necessarily  an  error,  as  the  result  given  in 
the  table  is  the  average  of  many  specimens  : 


NAME. 

W. 

W. 

Loss. 

SP.  G. 

D. 

EXERCISE  45.  —  Weigh  a  solid  less  dense  than  water,  e.  g.,  a 
piece  of  paraffine,  wood  or  cork  that  has  been  coated  with  oil  or 
hot  paraffine  to  make  it  water-proof.  Attach  to  it  a  sinker  and 
weigh  them  together  in  water.  Weigh  sinker  in  water.  From 
these  three  weights  find  the  specific  gravity  of  the  body  tested. 

EXERCISE  46.  —  Find  the  specific  gravity  of  a  liquid  by  means 
of  a  specific  gravity  bottle.  The  amount  of  water  the  bottle  will 
hold  is  usually  cut  in  the  glass,  and  need  not  be  tested  by  the 
pupils.  If  the  bottle  has  no  counterpoise,  balance  it,  when  per- 
fectly dry,  with  sand.  Fill  the  bottle  with  the  liquid  to  be  tested, 


64  LABORATORY  MANUAL. 

weigh,  and  calculate  the  specific  gravity.  In  filling,  be  sure  nv 
air  bubble  is  under  the  stopper  ;  wipe  the  outside  dry,  and  be  very 
careful  that  none  of  the  liquid  is  brought  in  contact  with  the 
scale-pan.  If  a  second  liquid  is  to  be  tested,  rinse  out  the  bottle 
with  a  little  of  this  liquid,  then  proceed  as  before. 

EXERCISE  47.  —  Obtain  a  narrow  U  tube  having  arms  about  50 
cm.  long.  Pour  into  it  water  until  it  stands  about  20  cm.  high  ic 
each  arm.  Next  pour  into  one  arm  some  liquid  that  will  not  mix 
with  water,  as  kerosene,  mercury,  etc.  Measure  the  vertical  dis- 
tances from  the  surface  where  the  liquids  meet  to  the  free  surfaces 
of  the  liquids.  In  this,  as  other  experiments,  measure  to  the  level 
of  the  upper  surface,  not  to  the  upper  edge  of  the  meniscus. 

From  your  measurements,  calculate  the  specific  gravity  of  the 
liquid  tested. 

Alternative :  —  Connect,  by  means  of  rubber  tubes,  to  each  arm 
of  a  Y  tube,  a  glass  tube  about  50  cm.  long.  Over  the  stem  of  the 
Y  slip  a  rubber  tube  about  20  cm.  long.  Unless  the  rubber  fits  the 
glass  closely,  wrap  with  soft  cord.  Insert  one  of  the  tubes  into  a 
beaker  of  water  ;  the  other,  into  a  beaker  containing  some  other 
liquid.  Cause  the  liquids  to  rise  in  the  glass  tubes  by  sucking  air 
out  of  the  rubber  tube.  Pinch  this  tube,  and  measure  the  height 
of  each  liquid  from  the  level  of  the  liquid  in  its  beaker.  Determine 
the  specific  gravity  as  before. 

EXERCISE  48.  —  Specific  Gravity  Determined  by  Meam>  of  a 
Hydrometer.  Prepare  a  stick  of  pine  1  cm.  square  and  25  cm.  long. 
Bore  a  hole  in  one  end  and  force  bullets  into  it  until  the  rod  will 
float  in  water  in  a  vertical  position  and  with  a  few  cm.  projecting 
above  the  surface.  Coat  with  hot  paraffine  to  make  it  waterproof. 
If,  before  coating  the  rod,  the  upper  part  of  it  is  graduated  in  mm., 
it  will  facilitate  the  work  with  the  instrument. 

Moat  this  hydrometer  in  water  in  a  suitable  vessel,  note  the 
depth  to  which  it  sinks.  How  much  water  does  it  displace  ?  How 
much  ought  it  to  weigh  ?  Now  float  in  some  other  liquid,  e.  g.,  salt 
water.  Note  the  depth  to  which  it  sinks,  and  from  your  two 
measurements  determine  the  specific  gravity  of  the  liquid  tested. 


66  LABORATORY  MANUAL. 


PNEUMATICS. 

EXERCISE  49.  —  Fit  to  a  two-liter  bottle  a  rubber  stopper 
through,  which  passes  a  short  glass  tube.  The  exact  volume  of  the 
bottle  should  be  determined  once  for  all  and  marked  on  the  bottle  ; 
this  may  be  done  by  weighing  it  when  empty  and  again  when  full 
of  water. 

Connect  this  bottle,  by  means  of  a  thick-walled  rubber  tube,  to 
an  air-pump  having  a  pressure  gauge.  Be  careful  to  make  all  joints 
air-tight.  Glycerine  is  useful  for  this  purpose.  Exhaust  the  air 
from  the  bottle.  Close  the  rubber  tube  with  a  pinch-cock,  and  at 
the  same  time  record  the  reading  of  the  gauge.  Now  weigh  bottle, 
stopper,  tube,  and  pinch-cock  ;  then  open  the  pinch-cock,  admitting 
the  air,  and  again  weigh.  The  difference  is  the  weight  of  the  air 
that  was  removed.  Its  volume  will  be  found  by  comparing  the 
reading  of  the  pressure  gauge  with  that  of  the  barometer  at  the 
time  of  the  experiment.  From  these  data  determine  the  specific 
gravity  of  air. 

EXERCISE  50.  —  (a)  Connect  to  one  arm  of  a  Y  tube,  by  means 
of  a  perforated  stopper,  a  glass  tube  50  cm.  long  and  about  15  mm. 
in  diameter ;  to  the  other  arm,  a  tube  about  5  mm.  in  diameter 
and  50  cm.  long.  Over  the  stem  of  the  Y  slip  a  short  rubber 
tube.  Introduce  both  glass  tubes  into  a  vessel  of  water.  Suck 
the  air  out  of  the  rubber  tube.  What  causes  the  water  to  rise 
in  the  glass  tubes  ?  How  do  the  heights  of  the  liquid  in  the  two 
tubes  compare  ? 

(b)  Measure  the  height  of  the  mercury  in  a  barometer  tube. 
What  supports  the  mercury  ?  What  change  would  it  make  in  the 
height  of  the  mercury  column,  if  the  cross-section  of  the  column 
was  1  scm.  ?  What  would  be  the  weight  of  such  a  column,  if  the 
specific  gravity  of  mercury  is  13.6  ? 

What,  then,  by  your  calculation,  is  the  pressure  of  air  per  scm.  ? 

EXERCISE  51.  — Work  the  lifting  pump  (or  force  pump),  noting 
carefully  the  position  of  the  valves.  Make  two  diagrams  of  the 
pump,  one  showing  the  position  of  the  valves  during  the  up 
stroke  of  the  piston,  the  other,  during  the  down  stroke  of  the 
piston. 


68  LABORATORY  MANUAL. 

EXERCISE  52.  —  Make  a  siphon  of  a  rubber  tube.  Compare  the 
velocity  of  water  when  the  outer  end  is  2  cm.  lower  than  the  sur- 
face of  water  in  the  vessel  with  the  velocity  when  the  same  end  is 
20  cm.  lower. 

EXERCISE  53. — Pour  a  little  mercury  into  a  Mariotte's  tube, 
and  adjust  by  shaking  until  the  mercury  stands  the  same  height  in 
both  arms.  It  is  evident  that  the  pressure  on  the  confined  air  is 
the  same  as  external  air  pressure,  or  one  atmosphere.  Also,  as  the 
expansive  force  of  an  elastic  body  is  equal  to  the  pressure  it  sup- 
ports, the  expansive  force  of  the  confined  air  is  one  atmosphere. 
This  may  be  expressed  in  g.,  or,  since  air  pressure  is  determined 
by  the  height  of  the  barometer,  we  may  represent  the  pressure  and 
expansive  force  of  the  confined  air  by  this  height.  Measure  the 
length  of  the  column  of  air  in  the  short  arm.  As  the  tube  should 
be  of  uniform  diameter,  this  length  may  represent  the  volume  of 
the  confined  air.  Pour  mercury  into  the  long  arm  until  the  vertical 
distance  between  the  mercury  surfaces  in  the  two  arms  is  about 
15  cm.  Carefully  measure  this  distance  and  the  height  (volume) 
of  the  air  in  the  short  arm.  Now  pour  in  mercury  and  measure 
as  before  until  you  have  made  five  or  six  measurements,  or  until 
the  tube  is  full.  Eecord  your  results  in  tabulated  form,  and  by 
comparing  them,  determine  the  law  connecting  the  variation  of 
pressure  with  variation  of  volume.  (See 'Exercise  21.  "Note  1.) 


70  LABORATORY  MANUAL. 


HEAT. 

In  the  work  in  heat  it  will  save  much  of  the  pupils'  time,  if  a 
vessel  is  provided  for  holding  water  that  may  be  warmed  to  the 
temperature  of  the  room,  from  which  pupils  may  take  water  for 
experiments  when  water  is  needed  at  that  temperature.  Also 
provide  a  reservoir  for  hot  water  so  time  will  not  be  taken  by  the 
pupils  for  heating  it.  If  access  can  be  had  to  a  steam-pipe  with  a 
faucet,  the  exhaust  steam  will  heat  water  very  quickly. 

EXERCISE  54.  —  Half  fill  a  4-ounce  bottle  with  water,  or  mer- 
cury. Carefully  note  its  temperature,  which  should  be  the  same 
as  that  of  the  room.  Close  the  bottle  with  a  stopper  ;  wrap  in 
several  thicknesses  of  paper  to  protect  from  the  heat  of  the  hand, 
and  shake  vigorously.  Note  change  of  temperature.  Shake  again 
for  a  longer  time,  and  again  note  the  temperature.  Evidently 
the  change  has  been  caused  by  the  shaking.  What  transforma- 
tion of  energy  is  this  ?  Give  three  other  examples  of  such  trans- 
formation. 

EXERCISE  55.  —  Pit  to  the  neck  of  a  4-ounce  bottle  a  stopper 
having  two  holes.  Half  fill  the  bottle  with  water.  Pass  through 
one  hole  of  the  stopper  the  stem  of  an  air  thermometer  bulb. 
Push  it  down  until  the  end  of  the  tube  is  well  under  water.  Now 
clasp  the  bulb  with  both  hands.  Explain  what  occurs.  Eemoving 
the  hands,  bring  a  flame  near  the  bulb.  Note  the  effect.  Let  the 
bulb  cool ;  then  apply  cold  water,  —  i-f  possible,  ice.  What  effect  ? 
Explain.  Are  two  holes  in  the  stopper  necessary  ?  Why  ? 

EXERCISE  56.  —  Fill  an  8-ounce  flask  (wide  test  tube)  with  cold 
water.  Fit  closely  to  its  neck  a  stopper  through  which  passes  a 
tube  30  cm.  long,  having  a  bore  of  1  or  2  mm.  The  water  should 
stand  in  the  tube  1  or  2  cm.  above  the  stopper.  Clasp  the  flask 
closely  with  both  hands.  Note  effect  on  the  height  of  the  water  in 
the  tube.  Next  apply  carefully  the  flame  of  a  Bunsen  burner. 
Note  as  before.  If  you  watch  closely,  you  may  notice  that  just 
as  the  flame  is  applied,  the  water  sinks.  Can  you  account  for 
this  ?  What  general  effect  of  heat  upon  water  does  this  experi- 
ment show  ? 


72 


LABORATORY  MANUAL. 


EXERCISE  57.  —  To  Find  the  Coefficient  of  Linear  Expansion 
of  a  Solid :  Fit  up  apparatus  as  indicated  in  diagram.  This 
consists  (1)  of  a  tin  tube  60  cm.  long,  2.5  cm.  in  diameter,  open  at 
both  ends.  On  opposite  sides  of  this  tube,  near  each  end,  is  a  short 
tube  about  .6  cm.  in  diameter.  Near  the  middle  is  a  tube  about  1.5 
cm.  in  diameter,  into  which  a  thermometer  may  be  fitted.  Per- 
forated c.orks  are  fitted  to  each  end  of  the  tube.  Through  these 
passes  a  brass  tube  of  about  .6  cm.  diameter,  and  a  little  longer 
than  the  large  tube.  .  The  whole  is  mounted  so  that  one  end  of  the 
brass  tube  presses  against  a  screw  at  the  right,  the  other  against 
the  short  arm  of  a  lever  whose  long  arm  moves  before  a  ruler 


Fig.  11. 

fastened  vertically.  (2)  The  other  piece  of  apparatus  is  a  boiler 
for  generating  steam.  It  is  a  sheet-copper  cylindrical  vessel  15  cm. 
tall  and  10  cm.  in  diameter,  mounted  on  three  legs  which  raise  it 
about  20  cm.  The  legs  are  kept  from  spreading  by  a  flat  ring  of 
sheet  metal  connecting  them  at  the  bottom.  Leading  out  from 
near  the  top  is  a  tube  5  cm.  long  and  6  or  7  mm.  in  diameter, 
through  which  steam  may  be  carried  off  in  a  slightly  ascending 
direction  when  the  top  of  the  vessel  is  closed.  A  conical  top  30 
cm.  tall  fits  the  cylindrical  vessel  as  a  cover  fits  a  pail.  The  joint 
must  be  as  near  steam-tight  as  possible  so  the  top  of  the  cylinder 
is  not  wired,  but  is  left  flexible.  The  open  top  of  the  conical 


74  LABORATORY  MANUAL. 

portion  is  about'  2.5  cm.  in  diameter,  and  of  such  shape  as  to  be 
easily  closed  with  a  stopper.  A  side  tube,  similar  to  the  one  on 
the  cylindrical  portion,  leads  out  from  near  the  top  of  the  cone. 
These  two  admirable  pieces,  called  the  "Linear  Expansion  App." 
and  "App.  A,"  are  described  in  Hall  &  Bergen,  and  are  supplied 
by  most  dealers  in  laboratory  materials. 

Measure  the  brass  rod  when  cold,  and  note  reading  of  ther- 
mometer. Adjust  the  screw  so  that  one  edge  only  of  the  brass 
tube  touches  the  lever.  To  do  this  easily  it  will  be  well  to  have 
the  end  of  the  tube  beveled  slightly.  Now  boil  the  water  in  App. 
A  vigorously,  and  pass  the  steam  through  the  tin  tube.  The  two 
upper  openings  of  App.  A  are  of  course  closed.  Note  carefully 
the  rise  of  the  lever,  reading  to  quarters,  or  less,  of  mm.  When 
the  lever  ceases  to  rise,  note  temperature  of  steam,  i.  e.,  of  the 
brass  tube.  Measure  carefully  the  lengths  of  the  two  arms  of  the 
lever.  From  this  you  can  compute  the  amount  of  expansion  of  the 
tube.  Now  determine  the  ratio  of  expansion  for  one  degree  C.  to  the 
original  length  of  the  rod.  This  is  the  coefficient  of  expansion. 

EXERCISE  58.  —  Fasten  on  a  board,  as  indicated,  wires  20  cm. 
long,  of  copper,  brass,  German  silver,  iron,  etc.  Heat  at  A  with  a 
Bunsen  flame.  Now  beginning  at 
the  ends  of  the  wire  farthest  from 
the  flame,  slide  along  each  in  turn 
the  head  of  a  match.  Note  the 
point  where  the  match  is  ignited. 
Measure  the  distance  from  this 
point  to  the  flame,  and  so  make  a 
list  of  the  metals  used,  in  the  order 
of  their  abilities  to  conduct  heat. 
Or  :  Move  the  tip  of  a  finger  care- 
fully along  each  wire  toward  the 
flame  until  a  point  is  reached  which  is  uncomfortably  warm.  Mea- 
sure the  distance  from  this  point  to  the  flame,  and  thus  make  a  list 
of  the  metals  as  above. 

EXERCISE  59.  —  Fill  a  test  tube  nearly  full  of  water.  Incline 
it  and  apply  the  tip  of  a  flame  to  it  near  the  top.  Take  care  that 
the  flame  does  not  touch  any  part  of  the  glass  not  covered  inside 
by  the  water.  Hold  in  this  position  until  water  at  the  top  of  the 
tube  boils.  How  is  the  temperature  of  the  lower  end  of  the  tube 


76  LABORATORY  MANUAL. 

affected?      What  does  this  show  as  to  the  ability  of  water  to 
conduct  heat? 

EXERCISE  60.  —  Fill  a  large  beaker  with  water.  Stir  into  it  a 
little  fine  sawdust.  Now  gently  heat  the  beaker  at  the  bottom, 
close  to  one  side.  Watch  for  evidence  of  currents.  Explain  the 
cause  of  these  currents  from  your  results  in  Exercise  56. 

Make  a  rectangle  of  glass  tubing  10  cm.  X  15  cm.,  joining  the 
ends  by  using  rubber  tubes  and  a  Y  tube.  Fill  with  water  in 
which  has  been  stirred  a  little  fine  sawdust.  Cautiously  heat  one 
of  the  lower  angles  of  the  rectangle.  Note  and  explain.  Cool  one 
of  the  upper  angles  by  touching  with  a  piece  of  ice.  What  occurs  ? 

This  illustrates  how  water  circulates  in  buildings  warmed  by 
hot  water. 

Where  would  you  apply  heat  to  water  to  warm  it  most  quickly? 

In  what  way  does  the  method  of  heating  a  metal  differ  from 
that  of  heating  water  ?  The  first  is  called  Conduction  ;  the  second, 
Convection. 

EXERCISE  61.  —  "  Touch  paper "  is  made  by  dipping  strips  of 
unsized  paper  in  a  solution  of  saltpetre  (potassium  nitrate),  the 
paper  being  dried  before  using.  It  burns  with  no  flame  and  a 
great  deal  of  smoke.  Light  a  strip  of  this  paper,  and  by  means 
of  the  movements  of  the  smoke,  study  currents  of  air.  Eecollect 
that  smoke  is  made  up  of  solid  particles  which,  being  visible  and 
very  light,  serve  to  indicate  the  motions  of  the  "air.  Why  does  the 
smoke  rise  ?  Hold  it  under  the  mouth  of  an  inverted  battery  jar. 
Note,  and  explain  the  motion  of  the  smoke.  Light  a  piece  of 
candle  about  3  cm.  long ;  set  it  on  the  table,  and  set  over  it  a 
student-lamp  chimney.  Procure  a  strip  of  tin  that  will  slip  easily 
into  the  chimney.  Near  the  end  of  this  punch  two  holes,  through 
which  run  a  stout  wire  a  little  longer  than  the  diameter  of  the 
chimney.  Drop  the  strip  down  the  chimney.  This  will  divide  the 
chimney  into  two  passages.  The  candle  should  stand  under  one  of 
the  passages.  Hold  the  burning  paper  close  above  the  top  of  the 
other.  Note  and  explain,  as  before,  the  movements  of  the  smoke. 

EXERCISE  62.  —  Fill  a  beaker,  or  other  vessel,  with  cracked  ice 
that  has  been  washed  clean.  Insert  in  it  a  thermometer  until  the 
point  marked  0°  (the  instrument  is  supposed  to  have  the  Centi- 
grade scale)  is  just  above  the  surface  of  the  ice.  When  the  mer- 


78  LABORATORY  MANUAL. 

cury  column  has  come  to  rest,  record  the  reading.  In  reading  the 
thermometer  in  this  and  other  experiments,  be  careful  to  stand  in 
such  a  position  that  a  line  from  the  centre  of  the  eye  to  the  top  of 
the  mercury  column  will  strike  the  stem  of  the  thermometer  at 
right  angles.  Read  to  quarters,  and,  if  possible,  to  tenths  of  the 
smallest  division  on  your  instrument.  On  these  two  precautions 
will  depend  much  of  your  success  in  the  succeeding  work.  Now 
sprinkle  on  the  ice  a  little  salt.  What  effect?  This  shows  the 
importance  of  having  clean  ice. 

EXERCISE  63.  —  Fill  App.  A  with  water  to  the  depth  of  3  cm. 
Close  the  side  tube  of  the  cylinder.  Fit  a  stopper  to  the  top  of  the 
cone.  Pass  through  it  a  thermometer,  pushing  it  down  until  the 
point  marked  100°  is  about  2  cm.  above  the  stopper.  The  bulb, 
however,  must  not  come  within  less  than  2  or  3  cm.  of  the  water 
in  the  vessel.  Boil  the  water  several  minutes,  read  very  carefully 
your  thermometer,  and  record.  In  order  to  find  what  would  be 
the  boiling  point  of  your  thermometer  under  standard  conditions, 
consult  the  barometer.  The  boiling  point  will  be  raised  or  lowered 
one  degree  for  every  excess  or  deficiency  of  27  mm.  from  the 
standard  760  mm.  What  would  be  the  boiling  point  of  your 
thermometer  under  standard  pressure?  Do  not  boil  the  water 
violently  lest  the  steam  be  compressed.  Partly  close  the  escape- 
pipe  a  moment  with  a  cloth.  Note  the  effect  on  the  boiling  point. 
Now  lower  the  bulb  into  the  water  and  note  reading.  Dissolve  in 
the  water  some  salt.  What  effect  on  the  boiling  point  ? 

EXERCISE  64.  —  Fit  to  a  wide  test  tube  a  stopper.  Through 
it  pass  a  short  glass  tube.  Over  the  outer  end  of  this  slip  a  rubber 
tube  about  10  cm.  long.  If  it  does  not  fit  the  tube  tightly,  wrap  it 
with  soft  cord.  Boil  the  water  in  the  tube  a  few  seconds  ;  then, 
first  removing  the  flame,  pinch  the  rubber  tube,  invert  the  test 
tube  and  pour  on  it  cold  water.  This  condenses  the  water  vapor 
in  the  tube  and  so  lessens  the  pressure.  In  the  preceding  exercise 
you  noticed  how  an  increase  of  pressure  affected  the  boiling  point. 
What  do  you  now  find  to  be  the  effect  of  lowering  the  pressure  ? 

EXERCISE  65.  —  Boiling  Point  of  Other  Liquids.  Fit  to  a  wide 
test  tube  a  stopper  having  two  holes.  Through  one  pass  a  ther- 
mometer, and  through  the  other  a  short  glass  tube  the  outer  end 
of  which  is  bent  at  right  angles.  Fill  the  test  tube  one-third  full 


80  LABORATORY  MANUAL. 

of  turpentine.  Cork  it  and  press  the  thermometer  down  into  the 
liquid.  Boil  by  means  of  a  sand  bath.  This  may  be  made  by 
filling  a  tin  pan  3  or  4  cm.  deep  by  10  or  15  cm.  in  diameter,  with 
fine  sand.  Immerse  the  lower  end  of  the  test  tube  in  the  sand  and 
apply  heat  from  below.  Boil  and  record  temperature.  Repeat 
with  ether.  But  apply  heat  by  introducing  the  test  tube  into  a 
vessel  of  water  heated  to  about  70°  C.  The  flame  must  first  be 
extinguished  to  avoid  danger  of  explosion. 

EXERCISE  66.  - —  Fill  a  beaker  with  water.  Record  its  tempera- 
ture and  that  of  the  salt  to  be  used.  Stir  into  the  water  a  handful 
of  the  salt,  —  better,  powdered  ammonium  chloride.  Again  note 
temperature.  What  effect,  if  any,  would  the  stirring  have  on  the 
temperature  ?  What,  then,  must  have  produced  the  change  noted  ? 

Mix  snow  and  salt  in  portions  of  about  2  to  1  (the  portions  are 
not  very  important).  Record  the  temperature.  In  the  first  part 
of  this  exercise  the  fall  in  temperature  was  due  to  the  dissolving 
of  the  salt.  Is  this  a  similar  phenomenon  ?  You  can  decide  this 
by  noting  which  melts  the  faster,  the  mixed  ice  and  salt,  or  the 
unused  ice  or  snow. 

Pour  on  the  back  of  your  hand  a  few  drops  of  ether.  Then, 
after  this  has  evaporated,  a  few  drops  of  water  of  about  the  same 
temperature  as  the  ether.  Which  feels  the  colder  ?  Which  evapo- 
rates the  faster  ?  As  both  were  at  the  same  temperature,  to  what 
may  the  difference  in  sensations  be  due?  Repeat  by  pouring  a 
little  of  each  liquid  in  turn  on  the  air  thermometer  of  Exercise  55. 

State  two  other  examples  of  cooling  by  evaporation. 

EXERCISE  67.  —  Apparatus :  A  calorimeter  ;  this  must  be  a 
vessel  of  thin  metal  about  6  cm.  in  diameter  and  12  cm.  tall, 
brightly  polished  ;  a  nickel-plated  "lemonade  shaker."  Into  this 
pour  water  to  the  depth  of  a  few  cm.  Cool  this  gradually  by 
adding  to  it  ice  or  snow,  —  finally  salt,  if  necessary.  Stir  the 
water  continually  with  a  thermometer  that  it  may  have  a  uniform 
temperature.  Watch  the  outside  of  the  bottom  of  the  vessel  for 
the  appearance  of  mist.  Note  the  highest  temperature  of  the 
water  (this  is  considered  the  temperature  of  the  metal,  and  hence 
of  the  air  in  immediate  contact  with  it)  at  which  the  mist  appears. 
Then  gradually  warm  the  vessel.  This  may  be  done  by  pouring 
into  it  a  few  drops  of  warm  water. 


82 


LABORATORY  MANUAL. 


Note  the  temperature  at  which  the  dew  unmistakably  begins  to 
disappear.  The  average  of  these  two  readings  gives  the  dew  point 
of  the  atmosphere.  Record  the  temperature  of  the  room,  the  tem- 
perature out  of  doors,  and  the  kind  of  weather.  How  much  must 
the  temperature  fall  to-day  in  order  that  it  may  rain  ? 

EXERCISE  68.  —  Determine  how  the  rate  at  which  a  body  cools 
is  affected  by  the  temperature  of  the  surrounding  medium,  e.  g.,  the 
air.  Select  for  the  body  whose  cooling  is  to  be  watched  a  ther- 
mometer bulb.  One  with  a  large  bulb  is  preferable. 

Eemove  the  top  from  a  tin  can  of  about  one  liter  capacity. 
Solder  to  this  can  three  strips  of  tin  by  which  it  may  be  tacked 
to  a  board  covering  the  top.  Through  this  cover  bore  a  hole,  — 
about  |  in.  in  diameter.  Blacken  the  inside  of  the  can  and  the 
under  side  of  the  cover.  This  may  be  done  by  painting  with  thin 
shellac  varnish  mixed  with  dry  lamp  black.  Immerse  the  can  in 
a  large  vessel  of  water.  Note  temperature  of  the  water,  which 
should  be  the  same  as  the  temperature  of  the  room.  Slide  the 
stem  of  the  thermometer  through  a  stopper  that  fits  the  hole  in 
the  board.  Warm  the  thermometer  to  about  80°  C.  by  holding  it 
above  (not  in)  a  flame.  Dry  the  bulb,  if  it  has  become  moist. 
Now  introduce  the  thermometer  into  the  hole  in  the  board  and 
adjust  so  the  bulb  shall  be  near  the  centre  of  the  can. 

The  temperature  of  the  air  now  around  the  bulb  is  assumed  to 
be  that  of  the  water,  which  you  have  noted.  This  will  remain 
practically  constant,  since  the  blackened  can  will  transmit  rapidly 
to  the  water  any  heat  received  by  the  air  from  the  thermometer, 
and  the  large  body  of  water  will  not  have  its  temperature  per- 
ceptibly raised.  Note  temperature  of  thermometer  when  finally 
adjusted,  and  at  the  end  of  each  succeeding  half-minute.  Eecord 
results  as  below  : 


(1) 

TEMPERATURE 
OF  AIR  IN  CAN. 

(2) 
TEMPERATURE 
OF  THERMOMETER. 

(3) 
FALL  OF  TEMP. 
OF  THERMOMETER. 

DIFFERENCE 
BETWEEN  1  &  2. 

State,  by  studying  last  two  columns,  a  rule  governing  the  fall 
in  temperature  of  a  body.   (Page  20,  Note  1.) 


84  LABORATORY   MANUAL. 

EXERCISE  69.  —  Fill  a  beaker  about  one-third  full  of  water,  and 
add  about  an  equal  amount  of  pounded  ice  or  snow.  Apply  the 
heat  of  a  Bunsen  flame.  Stirring  rapidly  with  a  thermometer  it 
will  be  noticed  that  the  ice  melts,  but  that  the  temperature  remains 
practically  the  same  as  long  as  there  is  any  considerable  portion  of 
ice.  Since  the  vessel  is  receiving  heat  from  the  flame  and  the  sur- 
rounding air,  the  heat  so  received  is  said  to  become  latent.  That 
is,  it  is  used  in  so  altering  the  cohesion  of  ice  that  it  becomes  liquid. 
Determine  the  amount  of  heat  that  becomes  latent  in  turning  1  g. 
of  ice  at  0°  to  water  at  0°.  For  our  unit  of  heat  we  will  take  the 
amount  of  heat  needed  to  warm  1  g.  of  water  1°  C.  A  heat  unit 
will,  of  course,  be  given  up  by  1  g.  of  water  in  cooling  1°  C. 

Weigh  a  dry  beaker  of,  say,  300  ccm.  capacity.  Pour  into  it 
about  200  g.  of  water.  Warm  the  water  to  about  50°  and  accurately 
take  the  temperature.  Drop  into  the  water  small  pieces  of  clear 
dry  ice  until  about  100  g.  have  been  added,  i.  e.,  until  the  level  of 
the  water  is  raised  about  one-half.  Stir  constantly,  but  not 
violently.  Note  the  temperature  when  the  last  piece  of  ice  is 
melted.  If  the  ice  has  not  all  melted  by  the  time  the  water  has 
been  cooled  to  5°,  dip  out  the  remaining  ice,  taking  as  little  water 
as  possible.  It  is  also  important  that  the  final  temperature  be 
about  as  much  below  the  temperature  of  the  room  as  the  tempera- 
ture at  the  first  was  above  it,  that  the  influence  of  the  surrounding 
air  may  be  disregarded.  If  the  ice  put  in  is  not  enough  to  do  this, 
more  may  be  added,  lie-weigh  to  determine  the  exact  amount  of 
ice  melted.  From  your  results  calculate  the  number  of  heat  units 
necessary  to  melt  1  g.  of  ice.  In  doing  this,  take  notice  that  the 
ice  is  not  only  melted,  but  also  warmed  to  the  final  temperature, 
and  that  the  amount  of  heat  used  in  both  these  operations  is  equal 
to  that  given  up  by  the  beaker  and  the  water  ;  —  the 
amount  of  heat  given  up  by  a  g.  of  glass  in  cooling 
one  degree  may  be  taken  as  .2  of  a  heat  unit.  If  a 
metallic  vessel  is  used,  of  course  its  specific  heat 
will  be  taken. 


EXERCISE  70.  —  Determine  the  Latent  Heat  of 
Water    Vapor.      Fit  to  the  lower  delivery  tube  of 
App.  A  (all  the  other  openings  being   closed)  an 
arrangement  indicated  in  the  diagram.    The  delivery 
rig.  13.  tube  should  reach  nearly  to  the  bottom  of  a  beaker 


86  LABORATORY  MANUAL. 

containing,  say,  250  g.  of  water  at  about  15°  below  the  temperature 
of  the  room.  Accurately  take  its  temperature.  Boil  water  in  App. 
A.  Wait  until  all  the  delivery  tube  is  hot  and  a  strong  jet  of 
steam  is  issuing  from  it ;  then  introduce  into  the  water  of  the 
beaker,  bringing  the  end  near  to  the  bottom.  Condense  enough 
steam  to  raise  the  water  to  about  the  same  number  of  degrees 
above  the  temperature  of  the  room  as  it  was  at  first  below  it. 
Why  ?  Then  remove  the  delivery  tube.  Stir  the  water  thoroughly 
and  note  its  temperature  very  carefully.  Again  weigh  and  deter- 
mine the  weight  of  steam  condensed.  Now  calculate  the  heat  given 
up  by  a  g.  of  steam  in  condensing  to  water  at  100°.  In  working 
this  problem  notice  that  the  heat  which  warms  the  water  and  the 
beaker  comes  from  two  sources,  —  the  condensation  of  the  steam 
to  water  at  100°,  and  the  cooling  of  the  water  thus  formed  from 
100°  to  the  final  temperature.  Allow  for  the  heating  of  the  beaker 
as  in  the  previous  exercise. 

EXERCISE  71.  —  In  our  last  two  exercises  we  have  made  use  of 
the  fact  that  glass  in  cooling  furnishes  only  .2  as  much  heat  as  the 
same  weight  of  water  ;  or,  the  specific  heat  of  glass  is  .2. 

Determine  the  specific  heat  of  shot.  Place  in  the  dipper  of 
App.  A  500  g.  of  fine  shot.  Cover  the  dipper  with  a  piece  of  card- 
board in  which  is  a  small  hole  through  which  a  thermometer  may 
be  passed.  Pour  into  a  calorimeter  100  g.  of  water  cooled  to  6°  or 
8°  below  the  temperature  of  the  room.  Place  the  dipper  into  App. 
A,  which  should  be  filled  with  water  nearly  to  the  level  of  the 
bottom  of  the  dipper.  Close  the  side  tube  of  the  App.  Boil  the 
water,  stirring  the  shot  continually.  When  the  shot  has  reached 
the  temperature  of  100°,  pour  it  quickly  into  the  calorimeter. 
Stir  thoroughly  and  when  sure  the  temperature  no  longer  rises, 
record  reading  of  thermometer. 

By  a  similar  method  to  the  one  used  in  the  two  previous  experi- 
ments, calculate jthe  heat  yielded  by  1  g.  of  shot  cooling  1°,  noticing 
that  in  this  case  the  vessel  containing  the  water  is  of  metal  and  its 
specific  heat,  if  brass,  is  about  .09.  If  the  vessel  is  only  partly 
filled,  consider  that  only  so  much  as  is  below  the  water  line  is 
warmed. 


88  LABORATORY   MANUAL. 


ELECTRICAL  ENERGY  FROM  FRICTION. 

EXERCISE  72.  —  Rub  one  end  of  a  warm  glass  tube  with  a  warm 
piece  of  silk,  and  hold  the  rubbed  end  near  some  bits  of  tissue 
paper  and  some  small  pieces  of  pith.  Note  carefully  what  follows. 
Repeat  several  times  to  be  sure  of  results.  Now  rub  a  piece  of 
sealing-wax  with  dry,  warm  flannel  and  do  as  you  did  with  the  glass 
rod.  Eesults  ?  The  peculiar  action  manifested  by  these  rubbed 
bodies  is  due  to  electrification  produced  by  the  rubbing.  Did  you 
do  work  in  producing  electrification  ?  The  kind  developed  on  the 
glass  rod  is  called  positive  electrification,  and  that  on  the  sealing- 
wax,  negative  electrification.  Bodies  exhibiting  these  phenomena 
are  said  to  have  charges  of  electrification  upon  them. 

Make  a  paper  stirrup  large  enough  to  allow  the  rod  or  the  wax 
to  pass  into  it  easily,  and  suspend  it  by  a  silk  thread.  Rub  the 
rod,  and,  without  touching  the  rubbed  end  with  the  hand,  place  it 
in  the  stirrup.  Now  rub  another  glass  rod  and  hold  it  near  the 
rubbed  end  of  the  suspended  rod.  What  result  do  you  get  ?  Try 
it  several  times.  Do  the  same  with  two  sticks  of  sealing-wax,  and 
record  results. 

Now  hold  a  rubbed  glass  rod  near  to  a  suspended  stick  of  wax 
on  which  there  is  a  charge  of  negative  electrification,  and  record 
results.  Hold  a  rubbed  stick  of  wax  near  a  suspended  glass  rod 
011  which  there  is  a  charge  of  positive  electrification,  and  record 
results.  State  the  law  of  action  between  charges  of  electrification 
covering  the  phases  shown  by  the  above. 

EXERCISE  73.. —  Suspend  a  pith-ball  by  a  silk  thread.  Call 
this  an  electroscope.  Develop  positive  electrification  and  hold 
the  rubbed  body  near  the  pith-ball.  Note  fully  the  action  of  the 
ball.  If  the  ball  is  attracted  to  the  rod  and  then  flies  away 
from  it,  try  to  touch  the  ball  with  the  rod,  and  account  for  the 
result.  Now  rub  the  stick  of  wax,  and  hold  it  near  the  ball. 
Are  the  results  the  same  ?  Why  ?  Rub  the  wax,  hold  it  near  the 
ball,  and  after  the  ball  has  touched  it  and  swung  away,  bring  a 
rubbed  glass  rod  near  the  ball,  and  account  for  results.  Hold  an 
unelectrified,  or  neutral,  body  near  a  positively  charged  electro- 
scope, and  then  near  a  negatively  charged  electroscope.  What  is 
the  action  in  each  case  ?  If  a  body  has  electrification  on  it,  how 
would  you  proceed  to  find  its  kind  ?  Can  electrification  do  work  ? 


90  LABORATORY  MANUAL. 

EXERCISE  74.  —  We  have  seen  that  certain  bodies,  upon  coming 
in  contact  with  an  electrified  body,  become  electrified.  Mount  two 
metallic  shells  or  balls,  such  as  are  used  on  the  ends  of  curtain  rods 
(or  two  potatoes  may  be  used)  on  some  short  sticks  of  sealing-wax 
so  that  they  will  be  on  about  the  same  level.  Some  pith-ball 
electroscopes  can  be  charged,  —  one  with  negative,  and  the  other 
with  positive  electrification,  to  be  ready  for  any  tests  you  wish  to 
make,  remembering  that  the  repellent  action  is  the  true  test  for 
the  kind  of  charge  on  a  body.  Hold  a  rubbed  glass  rod  near  one 
of  the  shells,  having  placed  the  two  shells  in  contact,  and  approach 
the  shells  from  the  opposite  direction  with  the  positively  charged 
electroscope.  Result  ?  Repeat.  Result  ?  Slowly  remove  the 
glass  rod  and  note  the  action  of  the  electroscope.  Test  the  two 
shells  for  electrification.  Result  ?  Touch  the  shells  and  the  pith- 
ball  with  your  hand.  Now  re-charge  the  pith-ball  electroscope 
positively.  Hold  the  electrified  rod  as  before,  being  careful  that 
no  sparks  pass  to  the  shells,  and  be  sure  the  shells  touch,  and 
again  bring  the  pith-ball  up  as  before.  While  the  rod  is  held  near 
the  first  shell,  carefully  remove  the  shell  and  the  rod.  Test  the 
two  shells  for  the  kinds  of  electrification.  Results  ?  This  process 
is  called  Charging  by  Induction. 

EXKKCISE  75.  —  Place  the  two  insulated  shells,  having  pith-ball 
electroscopes,  about  a  foot  apart,  and  connect  them  by  a  stick  of 
wax,  as  shown  in  the  sketch.  Hold  an 
electrified  rod  near  or  against  one  shell 
and  note  whether  the  wax  conveys  the 
electrification  to  the  other  shell.  Try,  as 
connectors,  a  glass  rod,  a  ruler,  a  silk 
thread,  a  piece  of  rubber  tubing,  and  a 
piece  of  metal,  and  then  try  a  wet  silk 
thread.  Make  a  list  of  those  substances 

which  allow  electrification  to  pass  to  the  second  shell.  These  are 
called  conductors ;  those  which  do  not  allow  the  electrification  to 
pass  are  called  non-conductors.  Name  them.  Why  do  the  shells  rest 
on  wax  ? 

EXERCISE  76.  —  Work  an  electric  machine,  and  slowly  separate 
the  knobs  in  front  of  the  wheels.  Do  you  notice  any  difference  in 
the  frequency  of  the  sparks  when  the  knobs  are  near  or  far  apart  ? 
If  you  get  a  large  spark,  it  indicates  a  large  charge  of  electrifica- 


92  LABORATORY  MANUAL. 

tion.  Should  you  desire  to  increase  the  electrification  on  the 
machine,  how  would  you  have  to  affect  the  air  space  through  which 
the  spark  passes  from  one  knob  to  the  other  ?  In  some  respects 
this  action  of  electrification  very  much  resembles  liquid  pressure. 
If  it  is  desired  to  have  great  water  pressure  throughout  the  city, 
the  pipes  through  which  the  water  passes  must  be  strong ;  and  if 
we  desire  to  have  a  great  charge  of  electrification  on  a  body,  we 
must  increase  the  air  space  between  that  body  and  any  other  body 
to  which  the  electrification  might  pass.  The  term  used  in  regard 
to  electrification,  as  pressure  is  in  liquids,  is  potential.  A  body  is 
said  to  be  at  high  potential  when  it  must  be  carefully  insulated  to 
keep  its  electrification  from  passing  to  the  earth.  Bring  one  knob 
of  the  electrical  machine  near  an  insulated  shell  provided  with  a 
pith-ball  electroscope,  and  pass  some  tiny  sparks  onto  it.  Does  the 
number  of  sparks  increase  the  potential  of  the  shell  ?  The  quan- 
tity of  electrification  on  an  insulated  conductor  is  called  its  charge  ; 
what  effect  does  increasing  the  charge  of  a  given  conductor  have  on 
its  potential  ?  Charge  two  insulated  shells  provided  with  pith-ball 
electroscopes  with  positive  electrification  so  that  they  will  have 
different  potentials.  Now  bring  them  together  and  note  the  effect 
on  the  electroscopes.  Eepeat  several  times.  This  effect  will 
always  be  seen  when  two  bodies,  at  different  potentials,  are  con- 
nected by  a  conductor.  The  transfer  of  electrification  by  means  of 
a  conductor  is  called  a  current.  The  earth  is  said  to  be  zero  poten- 
tial. Positively  charged  bodies  would  send  a  current  to  the  earth, 
and  with  negatively  charged  bodies,  the  current  is  said  to  be  in  the 
opposite  direction. 

EXERCISE  77.  —  One  end  of  a  narrow  strip  of  board  should  rest 
on  an  insulated  support,  and  the  other  on  any  good  conductor,  such 
as  the  steam-pipe.  Some  small  screw-eyes  can  be  screwed  into  the 
under  side  of  the  strip  at  equal  distances,  say  60  cm.  ;  and  from 
the  screw-eyes  suspend  double  electroscopes  by  means  of  No.  36 
bare  copper  wire.  The  balls  should  all  be  of  one  size ;  the  wires  by 
which  they  are  suspended  should  be  the  same  length,  and  the  ends 
of  the  wires  should  be  tipped  with  wax.  Fasten  the  wires  with  a 
loop  at  the  upper  end  so  that  the  balls  will  be  free  to  swing  side- 
ways. Now  pass  sparks  from  an  electrical  machine  onto  the  strip 
at  the  insulated  end,  and  watch  the  balls.  Does  the  electrification 
pass  the  whole  length  of  the  strip  ?  Is  it  equally  distributed 


94  LABORATORY  MANUAL. 

throughout  the  whole  length  ?  Does  all  of  the  electrification  which 
starts  from  the  insulated  end  reach  the  other  end  ?  Whatever 
tends  to  prevent  the  transference  of  electrification  from  one  point 
to  another,  is  called  resistance.  Does  the  strip  offer  resistance  to 
the  passage  of  electrification  ?  Was  electrification  used  up  in  over- 
coming this  resistance  ?  What  is  that  which  can  overcome  resist- 
ance and  is  used  up  in  so  doing  ?  Does  electrification  seem  to  be  a 
form  of  energy  ?  What  kind  of  energy  is  electrification  when  it  is 
stationary  ?  When  it  moves  along  a  conductor  ?  Are  heat  and 
light  ever  produced  by  the  passage  of  electrical  charges  ?  What 
transformation  of  energy  has  taken  place  when  a  spark  is  seen  ? 

EXERCISE  78.  —  Whatever  tends  to  cause  a  transference  of 
electrification  from  one  place  to  another  is  called  an  Electro- 
Motive  Force,  — written  E.M.F.  A  difference  of  potential  between 
two  points  is  an  E.M.F.  Can  you  think  of  any  way  of  producing 
an  E.M.F.  without  an  expenditure  of  energy  ?  If  a  difference  of 
potential  causes  an  E.M.F.,  does  the  E.M.F.  probably  increase  with 
an  increase  of  difference  of  potential  ?  When  you  work  the  elec- 
trical machine,  one  discharging  knob  becomes  positively,  and  the 
other  negatively,  charged.  Is  the  E.M.F.  greater  or  less  between 
the  two  knobs  than  between  either  of  them  and  an  unelectrified 
body  ?  Try  it  by  holding  an  unelectrified  body  as  far  from  one  of 
the  knobs  as  the  knobs  are  apart,  and  note  the  action  of  the  spark. 
Results  ? 

EXERCISE  79.  —  In  the  preceding  exercises  you  have  found  it 
necessary  to  insulate  a  body  in  order  to  electrify  it.  Now  try  to 
electrify  an  uninsulated  body.  A  piece  of  paper,  say  5X8  in.,  is 
fastened  to  one  side  of  a  pane  of  glass  by  mucilage  on  the  corners 
of  the  paper.  Warm  the  glass  slightly  and  lay  it,  paper  side 
down,  on  the  table,  and  on  the  glass,  just  over  the  paper  before 
used,  place  another  similar  paper.  The  two  are  now  separated  by 
the  glass.  Rub  the  upper  paper  with  a  cat-skin  or  piece  of  sheet 
rubber.  Handle  the  glass  carefully  by  the  corners  and  bring  the 
paper  surfaces  successively  near  a  suspended  pith-ball.  Does  either 
paper  seem  to  be  electrified  ?  Hold  the  glass  by  one  corner,  and 
pull  off  the  loose  paper  by  the  corner  and  test  it  to  see  whether 
it  is  more  highly  electrified  now  than  it  was  when  on  the  glass. 
Test  the  other  paper  now.  Results  ?  Repeat  and  test  each  paper 
for  the  kind  of  electrification  on  it,  and  note  whether  or  not  the 


96  LABORATORY   MANUAL. 

two  have  about  equal  amounts  on  them.  Results  ?  How  did  the 
lower  paper  become  electrified  ?  An  instrument  like  this  which 
enables  us  to  accumulate  quantities  of  electrification  is  called  a 
condenser.  The  most  common  form  consists  of  a  glass  jar  instead 
of  a  glass  plate,  and  instead  of  paper  coats,  it  has  tinfoil.  The 
jars  so  made  are  called  Leyden  Jars,  and  the  process  of  charging 
them  with  electrification  is  just  the  same,  only,  instead  of  rubbing 
either  coat,  the  knob  in  the  cork  of  the  jar  is  connected  to  the 
inner  coat  of  tinfoil  by  a  little  chain,  and  sparks  from  the  prime 
conductor  of  an  electrical  machine  are  passed  on  to  the  knob,  while 
the  outer  coat  is  connected  with  the  ground,  either  by  standing  the 
jar  on  the  table  or  by  holding  it  in  your  hand.  Do  not  try  to  get 
a  very  strong  charge,  and  handle  the  jar  very  carefully.  (You 
would  better  get  the  assistance  of  your  instructor).  Charge  the 
jar  thus,  and  having  placed  it  on  a  glass  plate,  or  cake  of  paraffine, 
connect  the  outer  coating  to  the  knob  by  means  of  a  metallic  con- 
nector having  an  insulator  handle.  What  follows  ?  Why  ?  In  a 
few  seconds  connect  the  coats  again.  Results  ? 

If  there  was  more  of  one  kind  of  electrification  than  of  the 
other,  what  will  be  the  condition  of  the  jar  after  the  sparks  cease  ? 
Test  the  jar  by  approaching  it  with  an  electroscope.  Eesults  ? 


98  LABORATORY  MANUAL. 


CURRENT  ELECTRICAL  ENERGY. 

EXERCISE  80.  —  lu  your  previous  work  you  learned  that  if  two 
bodies  were  at  different  potentials  and  were  connected  by  a  con- 
ductor, there  was  a  transference  of  electrification  from  one  to  the 
other.  If  the  bodies  could  be  kept  at  different  potentials,  the 
transference- of  electrification,  or  current,  as  it  is  called,  would  be 
continuous.  Such  devices  have  been  found,  and  one  of  them  is 
now  to  be  studied. 

Make  a  weak  solution  of  sulphuric  acid  by  pouring  about.  2  ccm. 
of  the  acid  into  a  tumbler  two-thirds  full  of  water.  Stand  in  this 
solution  a  brightly  sand-papered  strip  of  zinc  10  cm.  by  4  cm. 
Note  carefully  for  a  few  minutes  any  changes  on  the  surface  of 
the  metal.  Also  stand  a  similar  strip  of  bright  sheet-copper  in  the* 
solution,  but  not  in  contact  with  the  zinc.  Is  there  any  marked 
change  on  its  surface  ?  Kemove  the  zinc,  and  while  the  surface  is 
still  wet  with  the  acid  solution,  spread  a  drop  of  mercury  on  the 
zinc.  (Do  not  let  your  rings  come  in  contact  with  the  mercury.) 
A  little  cloth  may  be  used  to  spread  the  mercury  so  that  the  entire 
surface  of  the  zinc  becomes  bright  like  a  mirror.  Coating  the  zinc 
thus  is  called  amalgamating  it.  Keplace  the  zinc  in  the  solution. 
Does  the  same  action  appear  on  its  surface  ?  Keep  the  bottoms  of 
the  metals  apart,  but  lean  the  tops  together,  and  watch  the  surfaces 
for  the  evolution  of  bubbles.  Results  ? 

EXERCISE  81.  —  To  make  it  more  convenient,  you  can  use  a 
strip  of  pine  about  2  cm.  square,  and  long  enough  to  rest  across 
the  tumbler,  and  fasten  the  metals  on  opposite  sides  of  the  pine  by 
driving  through  each  strip  a  tack,  around  whose  head  is  wound  the 
bare  end  of  a  piece  of  No.  24  insulated  copper  wire.  Having  thus 
arranged  the  metals,  stand  them  in  the  acid  solution  and  press 
together  the  two  free  ends  of  the  wire,  being  careful  to  have  them 
scraped  bright  with  a  knife  or  sand-paper.  Is  any  action  mani- 
fested on  either  of  the  metal  plates  ?  Separate  the  wires  Result  ? 
Repeat  several  times,  and  record  results.  Connecting  the  wires 
thus  is  called  closing  the  circuit,  and  separating  them  is  called 
breaking  the  circuit. 

Examine  two  of  the  strips  which  have  been  used  for  some  time, 
and  tell  which  one  of  them  the  acid  solution  has  acted  on  the  more 


100  LABORATORY   MANUAL. 

vigorously.  This  "plate"  is  the  one  where  the  electrical  energy  is 
generated,  and  is  called  the  positive  plate.  It  is  at  a  higher  poten- 
tial than  the  other,  and  the  E.M.F.  causes  the  current  to  pass  from 
it  through  the  liquid  to  the  other,  or  negative  plate,  whence  the 
current  passes  through  the  wire  (external  path)  back  to  the  first 
plate.  Name  in  order  the  substances  through  which  the  current 
passes  in  making  a  complete  circuit. 

The  free  end  of  the  wire  connected  to  the  negative  plate  is 
called  the  positive  electrode ;  while  the  free  end  of  the  wire  con- 
nected to  the  positive  plate  is  called  the  negative  electrode.  A 
vessel  containing  a  pair  of  metals  arranged  similarly  to  the  one 
you  have  used,  and  having  in  it  a  solution  which  will  act  chemi- 
cally on  one  of  the  metals,  is  called  a  Galvanic  or  Voltaic  cell  or 
element.  When  several  cells  are  used  and  are  connected  with  one 
another,  they  constitute  a  Galvanic  battery.  (A  single  cell  is 
sometimes  called  a  battery.) 

Caution :  Never  twist  the  ends  of  wires  when  you  make  connec- 
tions ;  use  double  wire  couplers.  When  you  are  through  with  the 
pieces  of  wire,  wind  them  carefully  onto  a  spool  so  as  to  keep  them 
from  getting  twisted  or  tangled. 

EXERCISE  82.  —  There  are  many  kinds  of  Galvanic  cells,  and 
among  them  are  some  two-fluid  cells,  one  of  which  is  made  as  fol- 
lows :  use  for  materials  a  strong  glass  tumbler,  about  10  cm.  tall, 
and  7  or  8  cm.  wide  ;.  a  cup  of  unglazed  porcelain,  about  10  cm. 
tall,  and  4  or  5  cm.  wide  ;  about  half  a  liter  of  dilute  sulphuric 
acid  (20  parts  in  volume  of  water  to  1  part  in  volume  of  concen- 
trated acid)  ;  about  half  a  liter  of  a  saturated  solution  of  copper 
sulphate  ;  a  piece  of  zinc  10  cm.  long,  2.5  cm.  wide,  and  .5  cm. 
thick,  having  about  50  cm.  of  No.  20  insulated  copper  wire  fastened 
firmly  to  it ;  a  piece  of  sheet-copper  10  cm.  square,  and  having  a 
similar  wire  fastened  to  it. 

Put  the  zinc  into  the  porous  cup,  and  then  pour  in  dilute  acid 
until  the  cup  is  full  to  a  level,  2  cm.  below  the  top.  Now  put  the 
cup  into  the  tumbler  and  pour  into  the  tumbler  the  sulphate  of 
copper  solution  till  the  tumbler  is  full  to  a  level  with  the  acid  in 
the  porous  cup.  Kemove  the  zinc  and  amalgamate  it  with  mercury 
and  replace  it  in  the  porous  cup.  Put  the  sheet-copper,  bent  into 
a  circular  form,  into  the  sulphate  of  copper  solution.  Put  the  cell 
in  circuit  with  a  galvanometer,  and  note  the  deflection  when  the 
needle  comes  to  rest,  and  then  while  the  current  continues  to  pass 


102  LABORATORY  MANUAL. 

through  the  galvanometer,  note  the  deflections  for  periods  of  5  min. 
each  for  30  min.,  if  time  permits.  Results  ?  Compare  with  results 
obtained  when  a  single  fluid  cell  is  so  used.  (You  may  need  the 
aid  of  your  instructor  in  using  the  galvanometer.) 

EXERCISE  83.  —  Among  the  two-fluid  cells  the  most  common  is 
the  so-called  Gravity  cell.  It  consists  of  a  zinc  plate,  a  copper 
plate,  a  solution  of  zinc  sulphate,  and  a  solution  of  copper  sulphate, 
a  jar  to  hold  the  solutions  and  plates.  The  copper  plate  is  placed 
in  the  bottom  of  the  jar,  and  is  covered  with  a  saturated  solution 
of  copper  sulphate  ;  and  from  this  plate  is  extended  up  out  of  the 
jar  a  copper  wire  covered  with  hard  rubber.  Then  a  solution  of 
zinc  sulphate  is  carefully  poured  in,  and  in  a  few  minutes  is  seen 
to  be  resting  on  the  copper  sulphate  solution.  The  zinc  plate  is 
now  suspended  from  the  top  of  the  jar  so  as  to  be  immersed  in  the 
light  colored  solution  at  the  top  of  the  jar,  care  being  taken  not  to 
let  it  down  into  the  blue  solution  ;  the  zinc  should  be  covered  with 
liquid,  —  water  may  be  added  to  get  this  result. 

When  the  plates  are  connected  by  wires,  some  chemical  changes 
take  place  in  the  molecules  of  the  solutions.  The  copper  sulphate 
solution  deposits  copper  on  the  copper  plate,  and  zinc  from  the 
zinc  sulphate  takes  its  place  ;  and  the  zinc  plate  then  gives  off 
more  zinc  to  produce  new  molecules  of  zinc  sulphate.  In  time  the 
copper  sulphate  will  be  all  used  up,  and  there  will  be  too  much  zinc 
sulphate  solution.  This  is  shown  by  the  liquid  all  becoming  clear. 
Some  of  the  clear  solution  must  then  be  poured  off,  and  a  handful 
of  crystals  of  copper  sulphate  be  poured  into  the  cell.  Compare 
the  strength  of  the  current  from  this  cell  with  that  of  the  other 
two-fluid  cell.  Kesults  ?  Why  called  a  Gravity  cell  ? 

EXERCISE  84.  —  We  cannot  see  the  current  when  a  battery  is 
in  action,  but  we  can  see  some  of  its  effects,  and  you  are  now  to 
study  some  of  them.  Use  a  battery  of  two  or  three  large  cells, 
having  them  connected  in  series,  i.  e.,  the  positive  plate  of  one  cell 
connected  to  the  negative  plate  of  the  next,  and  so  on,  and  then 
between  the  copper  wires  which  connect  the  last  two  plates  place 
a  short  piece  of  very  fine  iron  or.  platinum  wire.  Note  carefully 
any  change  in  color  of  this  fine  wire,  or,  if  its  color  does  not  change, 
carefully  touch  it  with  the  finger.  Eesults  ? 

Remove  the  fine  wire  and  connect  the  two  copper  wires  by  a 
coupler  ;  scrape  the  insulation  from  a  short  section  of  the  wire 
through  which  the  current  is  passing,  and  dip  this  bare  section 


104  LABORATORY  MANUAL. 

into  some  iron  filings.  Result  ?  Break  the  circuit,  and  note  the 
effect  on  the  filings.  Repeat  several  times  to  be  sure  of  what 
happens,  and  record  results.  What  is  here  shown  to  be  the  con- 
dition of  a  wire  carrying  a  current  of  electricity  ? 

Now  dip  the  free  ends  of  the  wires  coming  from  the  battery 
into  a  solution  of  potassic  iodide  (made  by  dissolving  a  few  crystals 
of  potassic  iodide  in  water,  and  then  stirring  it  into  a  thin  starch 
paste),  and  note  the  effect  at  the  places  where  the  wires  touch  the 
solution.  A  dark  or  purple  color  indicates  a  chemical  change  by 
which  iodine  is  set  free.  Do  you  get  this  result  ?  Name  the 
three  effects  of  current  electricity  you  have  seen  in  this  exercise. 

EXERCISE  85.  —  Wind  some  insulated  copper  wire  No.  24 
around  a  piece  of  soft  iron,  such  as  a  wrought  iron  nail.  The 

wire  should  be  about  1  in. 
long,  and  shoxild  have  about 
30  cm.  free  at  each  end  after 
winding  onto  the  iron  so  as 
to  give  freedom  of  movement 
after  connecting  the  free  ends 
of  the  wires  to  a  galvanic  cell.  (The  tumbler  cell  will  do.) 

Wind  two  as  shown  in  Fig.  15.  '  The  figures  show  the 
wire  on  the  side  next  to  you  as  you  wind  them.  Having 
connected  the  ends  of  (1)  as  shown  above,  dip  A  and  B  suc- 
cessively into  a  dish  of  iron  filings.  Results  ?  Approach  the 
north-seeking  pole  of  a  magnetoscope  with  A  and  then  with  B. 
Results  ?  What  is  the  nail  now,  and  what  can  you  say  about  end 
A  and  end  B  ?  Now  disconnect  from  the  cell,  and  again  dip  the 
ends  of  the  nail  into  filings.  Results  ?  Under  what  conditions  is 
a  piece  of  soft  iron  a  magnet  ?  If  you  imagine  yourself  swimming 
in  the  "  current  of  electricity  "  in  any  section  of  the  wire,  as  at  E, 
and  facing  the  soft  iron,  on  which  side  of  you  is  the  north-seeking 
pole  of  the  magnet  which  the  soft  iron  becomes  ?  Now  make  the 
same  trials,  and  answer  the  questions,  using  2,  and  record  your 
results. 

These  devices  are  called  electro-magnets  while  the  current 
passes  through  the  insulated  wires.  The  coil  is  called  a  helix, 
and  the  soft  iron,  its  core.  If  you  look  at  the  end  of  1,  as  at  A, 
where  the  current  enters  the  helix,  the  current  will  pass  aK>und 
the  core,  as  is  shown  in  3.  Compare  this  direction  with  the  direc- 
tion of  motion  of  the  hands  of  a  clock.  This  is  a  right-hand  helix. 


106 


LABORATORY  MANUAL. 


Looking  at  2  where  the  current  enters  at  C,  the  current  will  travel 
around  the  core,  as  is  shown  in  4.  Compare  this  direction  with 
the  direction  of  the  motion  of  the  hands  of  a  clock.  Frame  a  rule 
for  telling  the  poles  of  an  electro-magnet  when  you  know  the  direc- 
tion of  the  current  around  it. 

If  you  were  given  an  electro-magnet  and  could  not  see  the 
battery  to  which  it  is  connected,  how  could  you  tell  the  direction 
of  the  current  in  the  wire  ? 

EXERCISE  86.  —  You  will  recall  the  effect  on  a  freely  suspended 
magnet  when  another  magnet  was  brought  near  it.  From  what 
you  saw,  when  a  wire  carrying  a  current  was  dipped  into  filings, 
what  do  you  think  would  be  the  effect  on  a  freely  poised  magnet, 
should  a  wire  carrying  a  current  be  held  near  it  ?  Use  an  insulated 
copper  wire,  No.  24,  about  1  m.  long.  Connect  the  bare  ends  of 
the  wire  to  the  poles  of  a  cell  and,  remembering  that  the  current 
is  from  the  negative  plate  through  the  wire  to  the  positive  plate 
(from  copper  to  zinc),  hold  the  wire  so  that  a  section  of  it  will 
carry  the  current  from  the  north  to  the  south  near  a  delicately 
poised  magnet,  and  get  results  from  each  of  the  following  con- 
ditions :  (Do  not  turn  the  wire,  even  though  the  needle  should 
turn  when  the  wire  approaches  it.) 


Current 
going 
from 


'  1.  North  to  south  above  magnet. 

2.  South  to  north  below       " 

3.  South  to  north  above       " 

4.  North  to  south  below       " 


North -seeking 
pole  deflected 
towards  east 
or  west  ? 


Examine  carefully  the  above  results,  and,  thinking  of  yourself 
as  swimming  in  the  "  current  of  electricity  "  and  always  facing  the 
magnet,  on  which  side  of  you  will  you  find  the  north-seeking  pole 
of  the  magnet  deflected  ?  State  the  rule  clearly  and  briefly. 

EXERCISE  87.  —  If  the  wire  should  be  held  so  that  the  current 
would  pass  from  north  to  south  above  a  freely-poised  magnet,  the 


Fig.  16. 


north-seeking  pole  would  be  deflected  in  what  direction  ?     If  from 


108  LABORATORY  MANUAL. 

south  to  north  below,  in  what  direction  ?  If  now  you  were  to  hold 
the  wire  so  that  one  section  of  it  would  carry  the  current  from 
north  to  south  above,  and  another  section,  from  south  to  north 
below,  how  would  the  amount  of  deflection  of  the  north-seeking 
pole  compare  with  either  of  the  first-mentioned  cases  above  ? 
Try  it  (A).  Results  ?  Try  by  having  several  coils,  as  shown 
in  B,  and  compare  results  with  the  trial  in  J,  and  state  any 
inferences. 

Use  a  larger  battery  cell,  and  compare  results.  Can  you  think 
of  any  use  to  which  you  could  put  an  instrument  of  this  kind  ? 

Instruments  having  several  coils  of  wire  around  a  delicately- 
poised  magnet,  under  which  is  a  scale  graduated  in  degrees,  is 
called  a  galvanometer.  How  would  increasing  the  number  of  coils 
(other  things  being  the  same)  affect  the  deflection  ?  If  the  num- 
ber of  coils  remained  constant,  how  would  increasing  the  currenj; 
affect  the  deflection? 

EXERCISE  88.  —  A  tangent  galvanometer  is  now  to  be  used.  It 
should  stand  on  the  table  so  that  the  coils  and  the  magnet  are  par- 
allel to  each  other  ;  this  will  be  true  when  the  pointers  are  on  the 
zero  marks.  The  instrument  should  be  level  so  that  the  needle  is 
in  the  center  of  the  coils,  and  free  to  move.  There  should  be  no 
iron  or  magnets  near,  and  if  the  needle  tends  to  stick,  tap  the  base 
gently  before  taking  a  reading.  Be  careful  not  to  change  the  coils 
from  their  north  and  south  position  while  making  tests.  The  earth 
tends  to  keep  the  needle  in  a  north  and  south  position,  while  a  cur- 
rent passing  through  the  coils  tends  to  turn  the  needle  east  and 
west,  so  that  when  the  galvanometer  is  in  use,  the  needle  assumes 
a  position  which  is  the  resultant  effect  of  the  two  forces  acting 
upon  it.  (Some  galvanometers  have  what  is  called  an  astatic  needle, 
i.  e.,  a  combination  of  two  magnets,  one  being  placed  above  the 
other,  with  opposite  poles  near  each  other,  so  that  the  earth's  effect 
on  one  magnet  neutralizes  the  effect  of  the  other,  and  the  influence 
of  the  earth  is  thus  eliminated.  Such  galvanometers  are  much 
more  sensitive  than  the  tangent  galvanometers.) 

If  the  number  of  coils  used  in  the  galvanometer  be  constant,  a 
greater  deflection  of  the  needle  indicates  what  ?  Now  find  the 
deflections  produced  by  two  or  three  different  cells,  and  record  the 
results.  It  is  better  to  read  by  reversals,  i.  e.,  cause  the  current  to 
pass  through  the  coils  in  one  direction,  and  then,  after  noting  the 


110  LABORATORY  MANUAL. 

deflection,  cause  the  current  to  pass  in  the  opposite  direction,  and 
note  the  deflection.  The  average  of  the  two  is  the  true  reading. 
(To  reverse  the  current  in  the  galvanometer,  a  commutator  may  be 
used,  or,  have  two  wires,  about  30  cm.  long,  coming  from  the 
galvanometer,  and  on  the  free  ends  have  wire  couplers,  so  that 
connections  may  be  made  without  disturbing  the  instrument.) 
Now  find  the  deflections  produced  by  two  or  three  different  cells, 
and  record  results  ;  always  get  readings  accurately  to  one-fourth  of 
a  degree.  The  strengths  of  currents  are  not  proportional  to  the 
degrees  of  deflection,  but  are  proportional  to  the  tangents  of  the 
degrees,  i.  e.,  if  one  cell  gives  a  deflection  of  55°,  and  another  cell 
gives  a  deflection  of  35  °,  the  strengths  of  the  currents  are  not  as 
55  :  35,  but  as  tangent  55  to  tangent  35.  The  table  of  natural  tan- 
gents is  found  on  p.  150,  and  by  consulting  it  you  will  find 
tangent  55  to  be  1.428,  and  tangent  35  to  be  .700,  so  that  the 
strengths  of  the  currents  are  to  each  other  as  1.428  :  .700,  or  about 
as  2  :  1. 

EXERCISE  89.  —  Connect  one  plate  of  a  cell  with  one  post  of 
a  galvanometer,  and  from  the  other  plate  of  the  cell  run  a  wire 
about  30  cm.  long,  having  its  free  end  scraped  bright.  Connect 
a  similar  wire  to  the  other  post  of  the  galvanometer.  Now  press 
these  two  free  bright  ends  of  the  wire  against  or  into  each  of  the 
following  substances,  and  note  the  amount  of  deflection  made  in 
each  case,  so  as  to  determine  which  substances  allow  the  current 
to  pass  through  them  easily  :  —  iron,  brass,  copper,  wood,  glass, 
carbon,  silver,  paper,  pure  water,  dilute  sulphuric  acid  and  zinc. 
Tabulate  as  follows  :  — 


SUBSTANCE. 

GALVANOMETER  DEFLECTION. 

By  examining  the  list  and  the  deflections,  number  the  sub- 
stances in  the  order  of  the  power  to  resist  a  current  passing  through 
them. 

Why  is  copper  wire  generally  used  ?  Why  is  it  covered  with 
cotton  or  silk  ?  Why  is  it  necessary  to  remove  the  insulation  in 
making  connections  ? 


112  LABORATORY  MANUAL. 

EXERCISE  90. — You  have  seen  that  substances  differ  in  their 
ability  to  resist  the  passage  of  an  electrical  current  through  them. 
To  find  just  how  much  resistance  a  certain  object  offers  makes 
it  necessary  to  have  some  standard  of  measurement.  The  unit 
accepted  is  called  the  "  ohm,"  and  is  the  resistance  of  a  column  of 
mercury  106.3  cm.  in  height,  and  of  uniform  cross-section,  and 
having  a  weight  of  14.4521  g.  at  a  temperature  of  0°  C.  Instru- 
ments consisting  of  several  coils  of  wires  of  different  known  resist- 
ances varying  from  .1  to  500,  or  more,  ohms  are  now  prepared, 
and  by  using  such  an  instrument  together  with  a  galvanometer, 
you  can  determine  the  exact  amount  of  electrical  resistance  of  any 
conductor.  The  resistance  box  and  galvanometer  are  connected  to 
a  battery  or  cell  so  that  the  current  passes  through  all  of  them, 
thus  :  Fig.  17.  The  keys  of  the  resistance  box  are  all  placed  on  the 

buttons  marked  zero,  and  the  de- 
flection of  the  needle  noted.  Now 
introduce  more  and  more  of  the 
coils  into  the  circuit  by  moving 
the  keys  around  on  successive  but- 
tons, thus  causing  the  current  to 
meet  with  greater  and  greater 
K  resistance  to  its  passage.  How 

does  this  affect  the  current,  and 

what  effect  does  it  have  on  the  needle  ?  Now,  with  any  resistance, 
say  5  ohms,  read  the  deflection.  Break  the  circuit  and  then  place 
a  bar  magnet  on  the  table  about  20  or  30  cm.  from  the  coil,  and 
have  it  exactly  in  line  with  the  needle  when  its  pointer  is  on  the 
zero  mark,  having  the  north-seeking  pole  of  the  magnet  towards 
the  south-seeking  pole  of  the  needle.  Close  the  circuit  and  again 
note  the  reading.  Does  the  presence  of  the  magnet  increase  or 
decrease  the  sensitiveness  of  the  galvanometer  ?  Reverse  the  ends 
of  the  bar  magnet,  and  find  the  effect  upon  the  readings.  Vary 
the  distance  of  the  bar  magnet,  and  record  results. 

EXERCISE  91.  —  One  method  of  measuring  resistances  is  as  fol- 
lows :  The  object  whose  resistance  is  sought  is  placed  in  circuit 
with  a  battery  and  a  galvanometer,  and  the  deflection  noted.  (Use 
a  magnet  and  reduce  the  reading  to  about  45°.)  It  is  better  to 
read  by  reversals,  and  take  the  average  reading  of  several  trials. 
The  object  is  then  removed,  and  a  resistance  box  is  substituted  in 


114 


LABORATORY  MANUAL. 


its  place  and  resistances  are  introduced  in  the  box  till  the  galva- 
nometer gives  the  same  deflection  as  was  obtained  when  the 
former  reading  was  taken  (read  to  quarters  of  degrees),  when  the 
total  resistance  introduced  by  the  box  will  be  the  resistance  of 
the  object  tested.  Kecord  your  results.  This  is  called  determin- 
ing resistance  by  substitution.  By  the  process  just  described, 
determine  the  resistance  of  several  coils  of  wire  and  record  results 
as  follows  : 


NO.  OF  WlEE, 

B.  &S. 

DIAMETER. 

LENGTH. 

KIND  OF  WARE. 

KESISTANCE. 

1 

2 

22 
28 

.644  mm. 
.321  mm. 

10m. 
10m. 

Copper. 

3 
4 
5 

22 
28 

28 

20m. 
20m. 
10m. 

German  silver. 

From  the  above  table  and  the  results  you  obtained,  can  you  see 
a  definite  relation  existing  between  the  lengths  of  conductors  of 
the  same  diameters  and  their  resistances  ?  State  it.  How  do 
resistances  vary  in  regard  to  diameters  or  sectional  areas  of  wires 
of  the  same  length  ? 

The  results  from  (2)  and  (5)  indicate  what  in  regard  to  ma- 
terial ? 

EXERCISE  92. —  To  measure  the  resistance  of  a  battery  by 
means  of  a  tangent  galvanometer. 

The  battery  is  connected  to  the  galvanometer  so  that  the 
deflection  is  about  45°,  and  the  exact  deflection  is  noted.  Now  by 
means  of  a  resistance  box  introduce  resistance  till  an  angle  is 
obtained  whose  tangent  is  just  half  of  the  tangent  of  the  angle  of 
the  former  deflection.  (Consult  the  table  of  angles  and  their 
tangents  on  p.  151.)  The  resistances  offered  by  two  bodies  to 
a  current  are  to  each  other  inversely  as  the  tangents  of  the  angles 
of  deflection  on  the  tangent  galvanometer,  hence  the  resistance 
introduced  by  the  box  above  is  equal  to  the  resistance  of  the  coils 
used  in  the  galvanometer  and  the  resistance  of  the  battery. 

If  now  you  deduct  the  resistance  of  the  coils  used  in  the  galva- 
nometer from  the  resistance  introduced  in  the  box,  you  will  have 
the  resistance  of  the  battery.  For  example,  if  the  battery  and 
galvanometer  in  circuit  give  a  deflection  of  45°  the  tangent  of 


116 


LABORATORY  MANUAL. 


45°  is  1.000.  Half  that  tangent  is  .500.  Tangent  .500  is  that 
of  angle  27°.  The  resistance  introduced  to  bring  the  deflection  to 
27°  is  8.2  ohms.  The  resistance  of  the  galvanometer  coils  used  is, 
say,  1.5  ohms.  Then  8.2  — 1.5  =  6.7;  therefore,  the  resistance  of 
the  battery  is  6.7  ohms.  Measure  the  resistance  of  two  different 
cells  thus  and  record  your  results. 

EXERCISE  93.  —  Use  an  astatic  galvanometer  and  a  Wheatstone 
Bridge.     The  Wheatstone  Bridge  presents  one  of  the  best  known 


is. 


methods  of  quickly  and  accurately  measuring  resistances.  It 
works  on  the  following  principle  :  The  current  starting  from  the 
battery  reaches  the  point  marked  E  on  a  thick  square  of  wire  or  a 
strip  of  metal  ;  here  it  divides,  and  if  R  and  R  are  connected  by 
a  thick  copper  wire,  and  X  and  X  are  similarly  connected,  then 
equal  amounts  of  the  current  pass  over  E,  A,  L  and  E,  B,  L,  the  two 
amounts  uniting  at  L  and  passing  on  to  the  battery.  For  every 
point  in  E,  B,  L  there  is  a  point  in  E,  A,  L  having  the  same  poten- 
tial, hence  should  any  two  such  points  be  connected,  there  would 
be  no  current  through  the  connecting  medium.  A  and  B  are 
two  such  points,  being  equally  distant  from  E,  and  should  you 
insert  a  galvanometer  between  them,  there  would  be  no  deflection 
of  its  needle.  If,  now,  you  should  introduce  a  coil  of  wire  between 
X  and  X)  one  division  of  the  current  would  have  to  pass  through 


118  LABORATORY  MANUAL. 

the  path  E,  B,  w,  L,  and  would,  therefore,  meet  more  resistance 
than  it  would  by  passing  over  the  path  E,  B,  G,  A,  L,  so  that  part 
of  the  current  would  pass  through  the  galvanometer,  as  shown 
at  1.  If  you  should  remove  the  heavy  wire  between  R  and  R,  and 
put  it  between  X  and  X,  and  should  put  the  coil  w  between  R  and 
R,  the  process  would  be  reversed,  and  the  division  of  the  current 
going  over  the  path  E,  A,  L  would  subdivide  at  A,  part  going  over 
E,  A,  G,  B,  L,  thus  giving  a  reverse  deflection  in  the  galvanometer. 
Use  No.  16  or  18  wires  of  about  equal  lengths  to  make  connec- 
tions, and  set  up  the  apparatus  as  shown  in  the  diagram.  Have 
two  keys  for  closing  and  breaking  the  circuits,  one  between  the 
point  L  and  the  battery,  and  the  other  between  A  and  the  galva- 
nometer. (Always  close  the  battery  circuit  first.)  Close  the  cir- 
cuits and  adjust  the  switches  in  the  resistance  box  till  there  is  no 
deflection  of  the  galvanometer  needle.  The  resistances  of  the  wire 
coil  and  that  introduced  in  the  box  are  now  equal.  What  is  the 
resistance  of  the  wire  tested?  Test  several  different  coils,  and 
record  results. 

It  is  better  to  unplug  unequal  resistances  in  the  arms  of  the 
bridge  marked  (a)  and  (£);  for  example  10  ohms  in  (a)  and  1  ohm 
in  (b)  if  the  object  to  be  tested  is  one  of  small  resistance,  and  if  it 
is  one  of  great  resistance  unplug  10  ohms  in  one  arm  and  100  ohms 
in  the  other.  Be  sure  that  the  plugs  marked  (P)  are  in  firm  con- 
tact with  the  metal  strip  unless  the  resistances  marked  on  their 
spools  are  to  be  thrown  into  the  circuit. 

Close  the  circuits  and  adjust  the  switches  in  the  resistance 
box  till  no  deflection  of  the  galvanometer  needle  can  be  noticed. 
Then  since  it  is  the  principle  of  the  Wheatstone  Bridge  that  when 
the  above  condition  is  obtained  the  products  of  the  resistances 

be 
of  the  opposite  arms  are  equal,  e.g.,  a '. b  ::  c : d  or  d  =— ,  you  can 

find  the  numerical  value  of  (d)  which  will  be  the  resistance  sought. 
Find  the  resistances  of  several  coils  and  record  results. 

EXERCISE  94.  Wind  several  m.  of  insulated  copper  wire,  No.  18 
or  20,  around  a  pasteboard  cylinder  about  4  cm,  in  diameter,  and 
leave  about  2  m.  of  each  end  of  the  wire  unwound  so  as  to  make 
connections  with  an  astatic  galvanometer.  Place  some  books  on 
the  wires  near  the  galvanometer,  so  that  in  moving  the  coil  you 
will  not  move  or  disturb  the  galvanometer,  and  in  your  manipula- 
tion keep  the  coil  as  far  from  the  galvanometer  as  possible,  Slide 


120  LABORATORY  MANUAL. 

the  coil  suddenly  over  one  end  of  a  bar  magnet,  as  you  would  a 
ring  on  a  finger,  and  note  the  effect  on  the  needles  of  the  galvano- 
meter. Remove  the  coil  suddenly,  and  note  the  effect.  Repeat 
several  times,  to  be  sure  of  results.  Now  reverse  the  ends  of  the 
magnet,  and  do  as  before,  and  note  how  the  deflections  compare 
with  the  former  ones.  How  does  the  direction  of  the  deflection 
produced  by  sliding  the  coil  onto  one  end  of  the  magnet,  compare 
with  that  of  the  deflection  made  by  removing  it  from  the  other  end 
of  the  magnet  ?  Make  several  trials.  Result  ? 

Hold  the  coil  still  while  it  surrounds  the  magnet,  and  note 
whether  a  current  is  manifested  in  the  galvanometer.  'When 
the  coil  is  moving  on  or  off  the  magnet,  the  number  of  lines  of 
magnetic  force  passing  through  the  closed  circuit  of  the  wire  is 
varied,  i.  e.,  when  the  coil  passes  onto  the  magnet,  the  number  of 
lines  of  magnetic  force  passing  through  the  closed  circuit  of  wire  is 
being  increased,  and  while  the  coil  is  being  withdrawn,  the  number 
of  lines  of  force  is  being  diminished.  Will  either  process  cause  a 
current  ?  When  the  number  is  not  being  varied,  is  there  a  cur- 
rent ?  How  does  the  deflection  made  by  increasing  the  number  of 
lines  at  one  pole  of  the  magnet,  compare  with  decreasing  the 
number  of  lines  at  the  other  pole  ? 

How  could  you  arrange  a  combination  of  this  kind  to  make 
a  continuous  current  in  one  direction  ? 

This  is  the  plan  of  a  dynamo.  If  one  is  at  hand,  examine  with 
the  aid  of  your  instructor. 

EXERCISE  95.  —  Wind  about  half  a  meter  of  No.  30  insulated 
copper  wire  around  a  wrought  iron  nail,  and  leave  about  1  m.  of 
each  end  of  the  wire  free.  Suspend 
the  nail  by  its  "point,"  as  shown  in 
Pig.  19,  connecting  the  wires  to  a 
battery.  Make  a  similar  electro- 
magnet which  you  can  use  without 
suspending  it.  It  can,  however,  rest 
on  a  cylinder  so  as  to  be  near  B. 
Cause  the  current  to  pass  through  A 
successively  in  opposite  directions 
Flg- 19-  by  changing  the  connections  at  P 

and  P'.  (Simply  press  the  bare  ends  of  the  wires  from  A  onto 
the  posta  P  and  P'  and  time  the  connections  so  as  to  be  in  unison, 


122 


LABORATORY  MANUAL. 


if  possible,  with  the  motion  of  B,  if  B  moves.)     Do  you  get  motion 
without  touching  B  or  breathing  against  it  ?     Why  ? 

If  you  could  have  some  electro-magnets  arranged  on  an  axle  so 
that  their  ends  would  project  like  the  spokes  of  a  wheel  from  the 


Fig.  20. 

hub,  as  shown  in  Fig.  20,  and  have  the  poles  as  indicated,  what 
would  the  joint  action  of  (1)  on  (2)  and  (1)  on  (3)  cause  the  wheel 
to  do?  If  you  could  continually  reverse  the  currents  in  the 
"wheel"  electro-magnets  so  as  to  keep  the  poles  in  the  vicinity  of 
1  of  the  polarity  indicated,  what  would  happen  to  the  wheel? 
This  is  the  plan  of  an  electro-motor,  and  the  connections  with  the 
wheel  coils  are  made  through  the  axle. 


124  LABORATORY  MANUAL. 


SOUND. 

EXERCISE  96.  —  Suspend  a  solid  rubber  cord  4  or  5  mm.  in 
diameter,  and  4  or  5  m.  long,  from  a  hook  in  the  ceiling,  or  other 
high  support,  and  fasten  the  lower  end  to  a  hook  in  the 
floor  so  that  the  cord  will  be  subjected  to  a  very  slight 
tension.  Measure  the  length  of  the  cord  between  the  two 
supports.  (The  cord  may  be  held  to  the  floor  by  light 
pressure  with  the  foot.) 

With  a  pointer  or  ruler  tap  the  cord  near  the  bottom  ; 
a  short  wave  will  run  up  the  cord.  Continue  to  tap  the 
cord  at  regular  intervals,  timing  the  intervals  so  that 
the  whole  cord  will  vibrate  as  one  ventral  segment,  as  in 
Fig.  21. 

When  you  have  learned  to  keep  the  cord  thus  regu- 
(          larly  in  motion,  count  the  number  of  double  vibrations  in 
Fig.  21.      gQ  seconds.     Repeat  before  recording. 

Again,  tap  the  cord  at  a  much  faster  rate  so  it  will  vibrate  in 
two  ventral  segments,  with  a  node,  i.  e.,  stationary  point,  at  the 
middle  of  the  cord.  Coiint  the  number  of  vibrations  now  made  in 
30  seconds. 

Similarly,  make  the  cord  vibrate  in  three,  then  four  ventral 
segments,  and  determine  the  number  of  vibrations  in  30  seconds  in 
each  case.  Tabulate  the  results  of  the  four  tests  as  follows  :  — 


LENGTH  OF  SEGMENTS. 

NITMHKR  OF  VIBRATIONS 
i*  30  SECONDS. 

NUMBER  OF  VIBRATIONS 
IN  1  SECOND. 

1 

2 

8 

4 

What  relation  between  length  of  segment  and  number  of  vibra- 
tions in  a  given  time  ? 

If  on  the  water  four  waves,  each  30  feet  between  crests,  were 
to  pass  a  given  point  in  1  minute,  what  would  be  the  velocity  per 
minute  of  the  onward  motion  of  the  wave  ? 

In  your  experiment  each  segment  was  but  one-half  the  wave 
length.  Knowing  this,  find  the  velocity  of  the  wave  motion  per 
second  in  each  of  the  above  cases.  Increase  the  tension  and  notice 
any  effect  this  has  on  the  rate. 


126  LABORATORY  MANUAL. 

EXERCISE  97.  —  Cause  a  bell  to  sound  by  striking  it  with  a 
pencil  or  other  light  body  ;  and  while  it  is  sounding  suspend  a 
small  pith-ball  or  piece  of  cork  from  a  thread  so  it  rests  lightly 
against  the  edge  of  the  bell.  What  results  ?  What  seems  to  be 
the  condition  of  the  bell  while  sounding  ? 

Tap  a  large  tuning-fork  on  a  piece  of  wood,  and  while  sound- 
ing, invert  it  and  dip  the  prongs  about  1  mm.  into  water.  What 
results  ?  Sound  again,  and  suspend  the  pith-ball  against  the 
prongs. 

Stretch  a  short  piece  of  No.  28  brass  spring  wire,  or  violin 
string,  so  that  when  plucked  it  will  yield  a  musical  tone,  and 
while  it  is  sounding,  suspend  a  card  or  bit  of  paper  lightly  against 
the  wire.  What  results  ?  Why  ?  Tear  a  strip  of  paper  1  cm. 
wide  and  4  or  5  cm.  long ;  then  fold  it  so  as  to  make  a  V-shaped 
saddle,  technically  known  as  a  "rider."  Now  drop  your  rider 
astride  the  sounding  string.  What  results  ? 

Fasten  the  center  of  a  6-in.  square  pane  of  glass  securely 
between  two  corks  by  means  of  a  clamp.  The  edges  of  the  glass 
should  be  ground  or  filed  smooth.  A  brass  plate,  if  not  too  thin, 
vvill  answer  still  better.  Sprinkle  sand  evenly  over  the  surface  of 
the  plate,  being  careful  that  the  plate  is  horizontal,  then  draw  a 
violin  bow  across  the  middle  of  one  edge  of  the  plate  so  as  to  make 
a  musical  tone.  What  results  ?  What  must  be  the  condition  of 
the  plate  ?  Make  a  sketch  of  the  plate  after  sounding  a  few 
times. 

EXERCISE  98.  —  Hold  a  vibrating  tuning-fork,  whose  pitch  is 
not  lower  than  middle  C,  over  the  mouth  of  an  empty  hydrometer 
jar  about  15  or  16  in.  deep.  Does  the  sound  appear  louder  than 
when  the  fork  is  held  away  from  the  jar  ?  Again,  bring  the  fork 
close  to  the  mouth  of  the  jar,  and  pour  water  slowly  into  the  jar, 
listening  carefully  for  the  greatest  intensity  of  sound.  Try  several 
times  till  you  feel  certain  you  have  the  point  of  greatest  reenforce- 
ment ;  then  measure  the  length  of  the  column  of  air  from  the 
surface  of  the  water  to  the  mouth  of  the  jar. 

With  the  air  column  of  the  same  length,  rotate  the  vibrating 
fork  slowly  on  its  axis  above  the  jar,  and  describe  the  effect. 
Now,  starting  from  the  position  with  one  prong  above  the  other, 
rotate  through  one-eighth  of  a  turn,  and  state  the  effect ;  then 
through  another  eighth,  etc.,  till  a  half-turn  is  completed.  Call 


128  LABORATORY  MANUAL. 

these  positions  1,  2,  3,  4  and  5,  and  designate  the  loudcess  of  each. 
This  change  is  due  to  Interference  of  the  sound  waves  started  by 
the  two  prongs. 

The  length  of  the  air  column  giving  greatest  reinforcement, 
plus  .3  the  interior  diameter  of  the  jar,  is  one-fourth  of  the  wave 
length  of  your  fork.  Knowing  this,  and  the  number  of  vibrations 
made  by  your  fork  in  a  second,  determine  the  velocity  of  sound 
per  second. 

It  will  be  well  to  repeat  the  experiment  with  a  fork  of  different 
pitch,  and  compare  with  previous  velocity. 

EXERCISE  99.  —  To  the  bob  of  a  heavy  seconds  pendulum  a 
white  cloth  is  tied  so  its  vibrations  can  be  seen  at  a  distance. 
The  pendulum  is  made  to  swing  through  a  large  arc,  and  just  as 
it  passes  the  middle  of  its  arc,  a  person  strikes  a  sharp  rap  on  a 
board  with  a  mallet  or  hammer,  the  motion  of  the  hammer  also 
being  made  visible  by  a  white  cloth.  A  second  person  seeks  such 
a  distance  from  the  pendulum  that  the  sound  is  heard  just  as  the 
pendulum  crosses  the  middle  of  the  arc  on  its  return  swing.  An 
opera-glass  or  spy-glass  will  be  of  great  assistance.  Begin  with  a 
distance  of  850  or  900  ft.,  and  increase  till  the  proper  point  is 
found.  As  this  method  is  at  best  not  very  accurate,  great  care 
should  be  exercised  in  timing  the  blows  of  the  hammer  to  the 
swing  of  the  pendulum.  What  do  you  find  to  be  the  velocity  of 
sound  ?  If  there  is  a  wind,  exchange  the  positions  of  observer 
and  pendulum,  and  find  to  what  extent  the  wind  increased  or 
decreased  the  velocity  of  sound. 

EXERCISE  100.  —  If  a  whirling  table,  with  Savart  wheel  is  at 
hand,  hold  the  edge  of  a  card  against  the  teeth  of  the  wheel,  or 
hold  the  corner  of  the  card  so  it  will  rub  on  the  row  of  holes  in 
the  wheel  while  the  wheel  is  in  motion.  A  tone,  more  or  less 
shrill,  should  be  produced.  Vary  the  rate  of  the  wheel,  and  decide 
how  the  velocity  with  which  the  card  is  made  to  vibrate  affects 
the  pitch.  If  no  whirling  table  is  at  hand,  the  escapement  may  be 
removed  from  an  old  clock,  and  a  card  held  against  the  rapidly 
revolving  wheel. 

Hold  a  tube  immediately  over  the  outer  row  of  holes  of  a 
Savart  wheel,  and  blow  steadily  through  the  tube  so  that  when  the 
wheel  is  revolved,  the  air  in  passing  from  the  tube  through  the 
holes  will  be  broken  up  into  puffs,  and  produce  a  musical  tone. 


130  LABORATORY  MANUAL. 

How  does  the  speed  of  revolution,  and  the  consequent  number  of 
puffs  per  second,  affect  the  pitch  ?  Try  over  another  row  of  holes. 

Result  ? 

If  you  have  a  good  Savart  wheel,  it  will  be  interesting  to 
revolve  till  you  bring  the  pitch  in  unison  with  a  tuning  fork  ; 
then,  turning  regularly  at  this  rate  for,  say,  ten  seconds,  determine 
the  number  of  revolutions  made  by  the  crank  on  the  wheel  per 
second,  then  the  number  of  revolutions  made  by  the  Savart  wheel 
per  second,  and  lastly,  by  counting  the  holes  in  the  row  used,  the 
number  of  puffs  per  second  which  produced  the  tone. 

EXERCISE  101.  — ^Apparatus :  A  glass  tube  about  1.5  m.  long 
and  having  an  internal  diameter  of  about  4  cm. ;  a  brass  tube  or 
rod  about  2  m.  long  and  1  cm.  in  diameter. 

Fit  to  one  end  of  the  glass  tube  a  stopper.  Scatter  through  the 
tube  a  small  quantity  of  cork  dust.  Fasten  to  the  end  of  the  brass 
tube,  by  means  of  sealing-wax,  a  thin  cork  that  slides  closely  in  the 
glass  tube,  but  moves  with  slight  friction.  Push  this  into  the  glass 
tube  about  50  cm.  Lay  all  upon  a  table,  and  clamp  the  brass  tube 
at  its  middle  point  by  means  of  two  grooved  blocks  and  an  iron 
clamp  so  that  it  will  be  held  in  a  horizontal  position  and  in  line 
with  the  axis  of  the  glass  tube. 

Eub  a  piece  of  resined  leather  over  the  outer  end  of  the  brass 
tube.  A  shrill  sound  should  be  produced  and  the  cork  dust  be 
violently  agitated.  By  moving  the  glass  tube  forward  or  backward 
so  as  to  change  the  length  of  the  confined  air  column,  a  point  may 
be  found  where  the  sound  will  cause  the  cork  dust  to  divide  into 
segments  separated  by  nodes.  Find  the  average  length  of  these 
segments. 

Now  these  segments  show  the  way  in  which  the  air  in  the  closed 
tube  is  vibrating,  and  as  the  distance  between  two  nodes  in  a  vibrat- 
ing body  is  half  a  wave  length,  we  may  thus  find  the  wave  length 
in  air  of  the  sound  made  by  the  brass.  But  the  brass  tube  having 
a  node  in  the  middle  is  also  half  a  wave  length.  By  comparing  the 
wave  length  in  the  brass  tube  with  that  in  air,  determine  the  rela- 
tive velocities  of  sound  in  the  two. 

EXERCISE  102.  —  Apparatus :  Sonometer  with  brass  spring  wire 
Nos.  22  and  28,  B.  &  S.  gauge. 

Tune  two  No.  22  wires  in  unison  so  they  will  give  a  good  musi- 
cal tone;  probably  a  tension  of  15  to  20  Ibs.  will  be  needed.  In 


132  LABORATORY  MANUAL, 

sounding  a  wire,  pluck  it  in  the  middle,  and,  with  the  ear  close  to 
the  wire,  listen  for  the  fundamental  tone,  which  may  for  an  instant 
be  obscured  by  harsh  or  grating  overtones. 

Having  the  wires  in  unison,  place  a  bridge  under  one  a  few  cm. 
from  the  end,  pluck  the  longer  portion,  and  note  any  change  of 
pitch.  (It  will  be  best  to  press  the  wire  lightly  against  the  bridge 
so  as  to  limit  the  vibrations  to  the  part  under  consideration.) 

Now  move  the  bridge  till  you  find  a  length  of  wire  whose  tone  is 
an  octave  above  that  of  the  full  lengthed  wire.  Measure  now  the 
length  of  the  wires. 

Inasmuch  as  the  higher  tone  of  an  octave  is  produced  by  twice 
as  many  vibrations  per  second  as  the  lower  tone,  find  from  your 
results  the  relation  between  the  number  of  vibrations  per  second 
and  the  length  of  the  wire. 

With  same  wires,  apply  tension  of  6  Ibs.  to  one,  and  if  it  does 
not  produce  a  musical  tone,  place  the  bridge  under  both  wires  at 
such  a  point  as  will  yield  a  good  tone  in  the  first  wire.  Now  place 
a  tension  of  24  Ibs.  on  the  second  wire  and  determine  the  musical 
interval  between  the  tones  produced  by  the  two  wires.  What  rela- 
tion do  you  find  between  ratios  of  the  numbers  of  vibrations  per 
second  and  the  ratios  of  the  tensions  ? 

Substitute  a  No.  28  wire  for  one  of  the  No.  22  wires.  Place  a 
tension  of  8  Ibs.  on  each  wire,  and,  if  necessary,  shorten  both  wires 
equally  by  means  of  the  bridge  so  as  to  get  a  good  tone  from  the 
thicker  wire.  Compare  the  vibration  rates  of  the  two  wires.  Com- 
pare the  diameter  of  the  wires  (accurately  measured,  or  taken  from 
the  tables)  with  the  vibration  rates. 

Inasmuch  as  the  wires  are  of  equal  length  their  weights  will 
vary  as  their  cross-sections,  or  as  the  squares  of  their  diameters. 
Now  find  the  relation  that  exists  between  the  vibration  rates  and 
weights  of  the  wires. 

EXERCISE  103.  —  Tune  two  No.  22  wires  in  unison  with  tension 
of  15  to  20  Ibs.  While  one  is  vibrating  its  full  length,  touch  it 
lightly  with  a  feather  exactly  in  the  middle ;  listen  for  the  pitch  of 
the  resulting  tone.  Eepeat,  and  drop  a  little  paper  rider  upon  the 
middle  of  the  wire  just  after  touching  with  the  feather.  What 
results  ?  Try  several  times.  What  do  you  call  the  stationary  points  ? 

Now  touch  the  wire  while  sounding  its  full  length  at  a  point  one- 
third  its  length  from  the  end.  What  tone  results  ?  Kepeat,  and 


134  LABORATORY  MANUAL. 

place  two  riders,  one  at  the  point  touched,  and  one  at  the  same 
distance  from  the  other  end,  What  results?  Try  several  times. 
How  does  the  wire  seem  to  vibrate  after  touching  ?  These  higher 
tones  are  called  overtones,  or  harmonics.  What  harmonic  results 
when  the  wire  is  touched  at  one-fourth  its  length  ? 

Now,  while  the  wire  sounds  its  fundamental,  see  if  you  can 
distinguish  any  of  the  harmonics  without  using  the  feather.  Sound 
one  of  the  wires  vigorously,  then  stop  it  with  the  hand,  and  listen 
to  see  if  any  sound  issues  from  the  other  wire.  If  so,  what  tone  is 
it  ?  The  wires  must  be  tuned  exactly  in  unison  to  get  results. 

Two  mounted  tuning  forks  of  same  pitch  will  answer  also. 
Sound  one ;  stop  it,  and  listen  for  the  other.  Vibrations  thus  pro- 
duced are  called  sympathetic  vibrations.  Can  you  account  for  the 
way  in  which  they  are  started  ?  Now  change  the  tension  of  one  of 
the  wires,  or,  if  using  the  forks,  fasten  a  drop  of  sealing-wax  to  the 
prong  of  one,  and  see  if  you  can  get  the  sympathetic  vibrations. 
Explain. 

Now,  while  the  wires  or  forks  are  slightly  out  of  tune,  sound 
the  two  wires,  or  the  two  forks,  and  listen  for  beats,  i.  e.,  swelling 
and  diminishing  of  the  tone.  Count  the  number  of  beats  in  a 
given  time,  say  10  seconds.  Then  change  the  tension  still  more, 
or  increase  the  weight  of  wax  on  the  prong,  and  count  beats  again. 
How  can  you  make  use  of  beats  in  tuning  an  instrument  ? 

EXERCISE  104.  —  Use  tuning  fork  and  two  Y  tubes  connected 
as  in  the  figure.  A  is  a  rubber  tube  10  in.  long  ;  B  is  a  bent  glass 

tube  12  in.  long  capable  of  sliding 
inside  the  larger  rubber  tube  C, 
which  is  13  in.  long ;  D  need  not 
be  more  than  a  few  inches,  but  E 
should  be  3  or  4  feet  long. 

Let  one  person  hold  the  free 
Fig-  ^  end  of  the  tube  E  in  his  ear  while 

another  holds  the  vibrating  fork  at  the  open  end  of  D.  At  first 
A,  B,  C  should  be  drawn  out  full  length.-  Now,  seek  to  adjust 
the  length  of  A,  B,  C,  so  that  no  sound  is  heard  when  the  fork 
is  held  at  D.  Pinch  the  tube  A  shut ;  does  the  sound  return  ? 
Try  repeatedly.  Having  found  a  length  of  A,  B,  C,  such  that 
when  A  is  closed,  the  fork  is  heard,  but  silence  results  on  opening 
A,  find  the  difference  in  the  lengths  of  the  two  paths  from  F  to  G. 


136  LABORATORY  MANUAL. 

Compare  this  with  the  wave  length  of  this  fork  as  determined  in 
Ex.  99.  Try  to  account  for  the  silence  when  both  tubes  are  open. 
The  experiment  may  be  varied  by  connecting  both  ends  of  a 
rubber  tube  1.5  to  2  m.  long  to  the  arms  of  a  Ftube  ;  a  second 
rubber  tube  connects  with  the  other  arm  of  the  Y  tube.  While  the 
free  end  of  this  last  tube  is  held  to  the  ear,  the  base  of  the  vibrat- 
ing fork  is  touched  to  the  wall  of  the  first  tube  at  such  a  point  as 
will  produce  silence.  Find  the  difference  in  length  of  the  two 
parts  of  this  tube  and  compare  this  difference  with  the  wave  length 
of  the  fork  as  directed  in  the  first  part  of  this  experiment. 


138  LABORATORY  MANUAL. 


LIGHT. 

EXERCISE  105.  —  Shadows.  Mount  two  cardboard  screens 
about  3  cm.  square  and  20  cm.  square.  This  may  be  easily  done 
by  fastening  each  with  sealing-wax  on  a  heavy  wire  and  sticking 
this  into  a  square  block.  Interpose  the  small  screen  between  a  flat 
gas  jet  and  the  larger  screen,  and  adjust  so  the  entire  shadow  of 
the  first  falls  on  the  second.  The  flat  flame  may  be  conveniently 
obtained  by  unscrewing  the  tube  from  a  Bunsen  burner  and  screw- 
ing in  its  place  a  piece  of  gas  tubing  tipped  with  a  regular  gas 
burner. 

Describe  or  sketch  the  shadow.  Now,  make  a  slight  hole  in 
the  lighter  part  of  the  shadow  called  the  penumbra.  Look  through 
it  at  the  flame.  If  you  see  the  flame,  state  what  part  of  it.  Now, 
make  holes  in  the  darkest  part  of  the  shadow,  called  the  umbra ; 
next  in  the  line  separating  the  umbra  and  penumbra ;  other  places 
in  the  penumbra.  State  what  you  see  in  each  case.  From  your 
observations,  state  the  cause  of  the  umbra  and  the  penumbra. 

Next  turn  the  edge  of  the  flame  toward  the  small  screen. 
Sketch  the  shadow.  What  sort  of  shadow  would  be  made  if  the 
light  came  from  a  single  luminous  point  ?  Test  your  last  conclu- 
sion by  turning  the  light  very  low.  What  sort  of  shadow  have 
you  ?  What  difference  have  you  noticed  in  the  shadows  made  by 
an  arc  light  and  by  a  gas  light  ?  Account  for  the  difference. 

EXERCISE  106.  —  Mount  three  cardboard  screens,  3  cm.  (A), 
6  cm.  (B),  and  9  cm.  ((7)  square,  with  their  centers  the  same 
height.  Turn  a  gas  jet  very  low  and  set  it  about  20  cm.  from  A, 
the  center  of  the  flame  being  the  same  height  as  that  of  the  screen. 
Adjust  B  until  the  shadow  of  A  just  covers  it.  If  A  should  now 
be  removed,  it  is  evident  all  the  light  that  fell  on  it  would  then 
fall  011  B  ;  as  this  is  four  times  the  size  of  A,  the  light  on  B  would 
be  only  one-fourth  as  bright  (intense)  as  on  A.  Or,  representing 
the  brightness  (intensity)  of  the  light  on  A  by  1,  the  intensity  on 
B  will  be  .25. 

Measure  as  carefully  as  possible  the  distance  of  the  center  of 
each  screen  from  the  center  of  the  flame. 


140 


LABORATORY  MAtfUAL. 


Now,  take  a  new  distance  for  A  and  adjust  B  as  before.  Try 
A  with  C ;  B  with  C ;  record  your  results  as  below,  always  repre- 
senting the  intensity  of  light  on  the  screen  nearer  the  flame  by  1 : 


SCREENS  USED. 

INTENSITIES. 

DISTANCES. 

A  and  B 

m.             n. 

d.             D. 

A  and  S 

A  and  C 

A  and  C 

B  and  C 

B  and  C 

Comparing  the  results  in  the  last  two  columns,  state  a  law  con- 
necting the  variation  of  intensity  with  variation  of  distance. 

EXERCISE  107. — Make  a  Bunsen  photometer  as  follows:  Cut 
out  of  heavy  unsized  paper  a  circle  about  10  cm.  in  diameter  ;  drop 
in  the  center  of  it  a  little  hot  paraffine.  Warm  the  paper  gently 
until  the  paraffine  has  saturated  the  paper,  making  a  translucent 
spot  about  3  cm.  in  diameter.  Mount  this  as  the  screens  were 
mounted  in  the  last  Exercise.  Do  not,  however,  get  .any  wax  on 
the  paraffined  spot,  or  allow  the  wire  to  cross  it.  When  this  paper 
is  illuminated  more  brightly  on  one  side  than  on  the  other,  the 
opaque  ring  appears  brighter  than  the  translucent  center  on  the  side 
of  the  brighter  light,  while  on  the  darker  side  the  reverse  is  true. 
When  the  two  sides  are  equally  illuminated,  the  spot  has  almost  the 
same  appearance  as  the  rest  of  the  paper.  Mount  on  a  block  a 
candle,  on  another  block  four  candles  in  a  row  close  together.  Set 
•the  single  candle  about  50  cm.  from  the  paper  so  that  the  light  from 
the  flame  shall  fall  perpendicularly  on  the  center  of  the  paper. 
Similarly  on  the  opposite  side  set  the  row  of  four  candles  parallel 
to  the  paper,  and  move  them  until  the  paper,  as  tested  by  the  trans- 
lucent spot,  appears  exactly  the  same  when  viewed  at  the  same 
angle  on  each  side.  Record  distance.  Make  several  trials  and  take 
the  average.  Since  the  two  sides  of  the  paper  are  now  equally 
illuminated,  how  must  the  light  which  it  receives  from  each  candle 
of  the  four  compare  with  that  it  receives  from  the  single  candle  ? 
How  do  their  distances  compare  ?  What  law  does  this  approxi- 
mately verify  ? 

Compare  the  intensity  of  the  light  of  a  candle  with  that  of  your 
gas  flame.  Adjust  them  on  opposite  sides  of  your  photometer  until 


142  LABORATORY  MANUAL. 

they  illuminate  it  equally.  Measure  their  respective  distances,  and 
from  these  calculate  the  ratio  of  the  intensity  of  the  candle  light  to 
that  of  the  gas  light. 

EXERCISE  108.  —  Mount  two  cardboard  screens  about  15  cm. 
square.  Place  these  about  20  cm.  apart  and  parallel  to  each  other. 
Cut  in  the  center  of  one  a  hole  2  mm.  in  diameter.  Place  20  cm. 
from  this  a  broad  gas  flame.  Describe  the  image  on  the  second 
screen.  How  does  it  compare  in  size  with  the  flame  ?  Can  you 
account  for  its  being  inverted  ?  Move  either  the  flame  or  the 
screen.  What  change  in  the  image  ?  Why  does  not  the  flame 
imprint  its  image  on  all  the  surrounding  surfaces  ?  You  may 
answer  this  by  putting  in  the  place  of  the  screen  with  one  hole 
another  having  many  holes. 

EXERCISE  109.  —  Reflection  from  a  Plane  Mirror:  Through 
the  middle  of  a  large  sheet  of  paper,  say  50  cm.  square,  draw  a 
line.  On  this  line  and  parallel  with  it  stand  in  a  vertical  plane  a 
piece  of  looking-glass,  about  10  cm.  by  20  cm.  If  the  glass  is 
thick,  it  is  well  to  recollect  that  the  coated  surface  is  the  reflector. 
Stick  a  pin  upright  in  the  paper  in  front  of  the  mirror.  Sight 
along  the  edge  of  a  ruler  laid  flat  on  the  paper  at  the  reflection  of 
the  pin.  Draw  a  line  AB  along  this  edge  ;  produce  this  line  to 
intersect  the  line  of  the  reflecting  surface.  Draw  a  second  line, 
BC,  from  this  intersection  to  the  pin,  and  also  from  the  same 
intersection  draw  a  perpendicular  to  the  line  of  the  reflecting 
surface.  The  angle  between  the  line  from  the  pin  and  perpen- 
dicular is  called  the  angle  of  incidence;  the  other  is  the  angle 
of  reflection.  How  do  they  compare  in  size? 

Eeplacing  the  mirror,  sight  along  the  ruler  from  some  other 
point  and  draw  a  line,  EF,  as  before.  Produce  EF  and  AB  until 
they  meet.  This  point  is  the  apparent  position  of  the  image  of  the 
pin.  How  do  the  distances  of  the  pin  and  its  image  from  the 
reflecting  surface  compare  ? 

In  front  of  the  mirror  draw  a  diagram,  as  a  triangle,  the  outline 
of  a  face,  etc.  Place  the  pin  upright  at  each  of  the  angles  of  your 
diagram  in  turn  and  locate  the  position  of  the  image  of  each  point. 
Connect  the  points  as  located.  How  does  the  image  so  formed 
compare  in  size  with  the  diagram  ?  Since  the  image  in  a  plane 
mirror  has  no  real  existence,  it  is  called  virtual  or  imaginary. 


144  LABORATORY  MANUAL. 

EXERCISE  110.  —  Hold  a  concave  mirror  in  the  sunlight  so  that 
the  reflection  from  it  is  circular.  Move  a  piece  of  chalk  back  and 
forth  in  front  of  the  mirror  until  the  point  is  found  where  this 
circle  is  smallest.  This  point  is  called  the  principal  focus  of  the 
mirror.  Its  distance  from  the  mirror  is  called  the  focal  length  of 
the  mirror  ;  measure  it  carefully  and  record. 

Second  Method :  —  Stand  alongside  of  a  flat  gas  jet  a  cardboard 
screen,  both  being  in  the  same  plane.  Move  the  mirror  back  and 
forth  in  front  of  these  until  you  obtain  a  sharp  image  of  the  flame 
on  the  screen.  The  screen,  or  flame  is  now  very  nearly  at  the 
center  of  the  sphere  of  which  the  mirror  is  a  part.  What  is  the 
radius  of  this  sphere  ?  Half  this  is  the  focal  length  of  the  mirror. 
Kecord  it. 

(a)  Is  the  image  formed  on  the  screen  virtual  or  real  ? 

(b)  Erect  or  inverted  ? 

(c)  How  does  it  compare  in  size  with  the  object  ? 

Place  the  flame  between  the  center  of  curvature  (center  of  the 
sphere)  and  principal  focus  of  the  mirror,  adjust  a  screen  until  a 
sharp  image  is  formed.  Nofe  distances  of  object  (flame)  and 
image  from  the  mirror.  Describe  the  image  according  to  (a),  (b) 
and  (c)  above.  Without  moving  the  mirror,  let  flame  and  screen 
change  places.  Describe  image  by  (a),  (b)  and  (c). 

Points  near  a  lens  or  mirror  related  to  each  other,  as  the 
positions  of  the  flame  and  image  in  this  experiment,  are  called 
conjugate  foci.  Formulate  a  definition  for  them. 

Move  the  flame  nearer  the  mirror  than  the  principal  focus. 
Can  you  get  any  image  on  a  screen?  Describe,  as  before,  the 
image  in  the  mirror. 

Place  the  flame  at  any  point  in  front  of  a  convex  mirror,  and 
describe  the  image  obtained. 

EXERCISE  111.  — Place  a  bright  coin  on  the  bottom  of  a  pan. 
Stand  in  such  a  position  that  the  coin  is  just  hidden  by  the  side 
of  the  pan.  Without  moving,  watch  while  another  pupil  pours 
water  into  the  pan.  Note  the  depth  of  the  water  when  the  coin 
appears  to  you,  if  it  does  so.  Remembering  that  we  see  objects 
by  the  rays  of  light  passing  from  them  to  our  eyes,  and  that  the 
rays  from  the  coin  could  not  reach  the  eye  before  the  water  was 
poured  into  the  pan,  draw  a  diagram  showing  what  effect  the  water 
must  have  had  on  the  path  of  some  of  the  rays  from  the  coin. 


146  LABORATORY   MANUAL. 

EXERCISE  112.  —  Hold  a  bi-convex  lens  in  the  sunlight  so  that 
a  circle  of  light  is  cast  on  a  screen  behind  it.  Move  the  screen 
until  the  circle  is  the  smallest  possible  ;  this  is  the  point  to  which 
the  lens  by  refraction  converges  the  parallel  rays  of  the  sun,  and  is 
called  the  principal  focus  of  the  lens,  and  the  distance  of  this  point 
from  the  nearer  face  of  the  lens  is  the  focal  length.  Eecord  it. 
Obtain  on  a  screen  the  image  of  some  distant  object.  How  does 
the  distance  of  this  image  from  the  lens  compare  with  the  focal 
length  previously  found  ? 

Second  Method:  —  Adjust  a  bright  gas  flame  and  a  screen  on 
opposite  sides  of  a  lens  so  that  the  image  of  the  flame  is  cast  on 
the  screen,  and  both  are  at  the  same  distance  from  the  lens.  Both 
are  now  at  points  called  secondary  foci  of  the  lens.  The  principal 
focus  is  midway  between  the  secondary  focus  and  the  lens.  Record 
the  focal  length  of  your  lens.  Compare  the  image  and  object  in 
this  Exercise  according  to  (a),  (&)  and  (c)  of  Exercise  110. 

Place  the  flame  between  the  principal  and  secondary  foci  and 
move  the  screen  until  a  sharp  image  is  obtained.  Note  distance  oi 
screen  and  object  from  the  lens,  and  describe  the  image  as  before. 

Let  screen  and  flame  exchange  places.  Describe  image  as 
before. 

What  name  may  be  given  to  the  points  where  the  flame  and 
image  stand  ? 

Place  the  flame  nearer  the  lens  than  the  principal  focus.  Can 
you  get  any  image  on  the  screen  ?  If,  by  looking  through  the  lens 
at  the  flame,  an  image  of  the  flame  is  seen,  describe  it. 

Place  the  flame  anywhere  before  a  concave  lens  and  describe 
the  image. 

EXERCISE  113.  —  Mount  two  small  bi-convex  lenses  about  3  cm. 
and  5  cm.  focus,  and  4  cm.  diameter,  by  sticking  the  edge  of  each 
into  a  slit  in  a  large  cork.  Hold  some  small  bright  object  in  front 
of  and  near  to  the  lens  of  shorter  focus.  Adjust  behind  it  a  screen 
so  as  to  obtain  a  sharp  image  of  the  object.  Beyond  this  screen 
and  nearer  to  it  than  the  focal  distance  of  the  second  lens  place 
the  second  lens.  What  sort  of  an  image  will  a  convex  lens  pro- 
duce of  anything  nearer  to  it  than  its  principal  focus  ?  Now 
remove  the  screen,  and  look  through  the  second  lens  toward  the 
first.  You  will  now  likely  see  a  magnified  virtual  image  of  the 
true  image  formed  by  the  first  lens  ;  a  little  more  adjustment  of 


148 


L  A  B  OR  A  TOR  Y   MANUA  /,. 


the  lenses  may  be  necessary  to  make  this  image  distinct.  This  is 
the  principle  of  the  compound  microscope.  The  lens  nearer  the 
object  is  the  objective  ;  the  other,  the  eye-piece. 

Adjust  as  in  the  previous  experiment  two  lenses,  using  for  the 
objective  a  lens  of  40  cm.  focus  and  10  cm.  diameter,  and  for  the 
eye  piece  the  lens  of  3  cm.  focus,  and  4  cm.  diameter.  Obtain  on 
your  screen,  however,  the  image  of  some  distant  object ;  then  place 
your  eye  piece  in  the  proper  position  and  describe  the  result.  This 
shows  the  action  of  a  compound  telescope. 


AMERICAN  WIRE  GAUGE.     (B.  &  S.) 


No. 

DlAM. 
IN  mm. 

RESIST  ANC 

PER 
1000  FT. 

E  IN  OHMS 

PER 

1000m. 

No. 

DlAM. 

ix  nun. 

RESISTANC 

PER 

1000  FT. 

E  IN  OHMS 

PER 

1000  in. 

1 

7.34B 

.129 

.423 

21 

.723 

13.323 

43.699 

2 

6.544 

.163 

.534 

22 

.644 

16.799 

55.090 

3 

5.827 

.205 

.672 

23 

.573 

21.185 

69.486 

4 

5.189 

.259 

.849 

24 

.511 

26.713 

87.618 

5 

4.621 

.326 

1.059 

25 

.455 

33.684 

110.483 

6 

4.115 

.411 

1.348 

26 

.405 

42.477 

141.324 

7 

3.665 

.519 

1.702 

27 

.361 

53.563 

175.886 

8 

3.265 

.654 

2.145 

28 

.321 

67.542 

221.537 

9 

2.907 

.824 

2.702 

29 

.286 

85.170 

279.357 

10 

2.588 

1.040 

3.411 

30 

.255 

107.391 

352.242 

11 

2.305 

1.311 

4.300 

31 

.227 

135.402 

444.118 

12 

2.053 

1.653 

5.421 

32 

.202 

170.765 

540.109 

13 

1.828 

2.084 

6.835 

33 

.180 

215.312 

706.223 

14 

1.628 

2.628 

8.619 

34 

.160 

271.583 

890.822 

15 

1.450 

3.314 

10.962 

35 

.143 

342.443 

1133.213 

16 

1.291 

4.179 

13.706 

36 

.127 

431.712 

1416.015 

17 

1.150 

5.269 

17.282 

37 

.113 

544.287 

1785.261 

18 

1.024 

6.645 

21.780 

38 

.101 

686.511 

2250.756 

19 

.899 

8.617 

28.265 

39 

.090 

865.046 

2837.350 

20 

.812 

10.566 

34.656 

40 

.080 

1091.865 

3581.317 

The  resistances  given  are  for  pure  copper  wire  at  a  temperature  of  24°  C.   (75°  P.), 
Ordinary  copper  wire  lias  a  resistance  5  or  6  per  cent,  higher  than  pure  copper. 


150 


LABORATORY   MANUAL. 


FORMULAE  FOB  CALCULATING  AREAS  AND  VOLUMES. 

Area  of  a  circle  =  nil-,  it  =  3.1416,  R  =  radius. 
Volume  of  a  cylinder  =  area  of  the  base  X  the  altitude. 
Volume  of  a  sphere  =  |  ieDa,  D  =  diameter. 


TABLE  OF  SPECIFIC  GRAVITIES. 


Alcohol,  absolute 

0.806 

Kerosene 

0  810 

"        common  . 

:  0.833 

Lead,  sheet  

11.400 

Alum 

1  724 

Limestone 

3.-180 

Ashwood 

0.690 

Marble 

2.720 

Beeswax  

0.964 

Mercury 

13.596 

Brass,  cast  

8.400 

Milk  

1.032 

Coal,  bituminous... 

..1.270  to     1.423 

Oak,  red  

0.850 

"     anthracite  

..1.260  to     1.800 

"     white  

0.779 

Cork  

0.240 

Oil,  olive  

0.915 

Copper 

8.850 

"     turpentine 

0  870 

Feldspar  

2.600 

Paraffin  

0.820  to     0.940 

Galena  

7.580 

Pine,  white  

0.554 

German  Silver  

8.432 

"      pitch  

0.660 

Glass  

2.50  to     3.600 

Platinum  Wire.... 

21.530 

Gold  

19.360 

Quartz   

2.650 

Granite  

2.650 

Silver,  cast  

..10.400  to   10.510 

Gutta-percha  

0.966 

Steel  

7.816 

Ice  

0.917 

Sulphur,  native  .. 

2.030 

Iron,  cast  

7.230 

Sulphuric  Acid  .. 

1.840 

"      wrought 

7.780 

Tin,  cast 

7  290 

India-rubber  

0.930 

Vinegar  

1.026 

Iron  Pyrites  

5.000 

Zinc,  cast  

7.000 

LABORATORY  MANUAL, 


151 


TABLE  OF  TRIGONOMETRICAL  FUNCTIONS. 


BEG. 

TANG. 

SINE. 

DEC. 

TANGENT. 

SINE. 

DEG. 

TANGENT. 

SINE. 

1 

.017 

.017 

31 

.601 

.515 

61 

1.80 

.875 

2 

.035 

.035 

32 

.625 

.530 

62 

1.88 

.883 

8 

.052 

.052 

33 

.649 

.545 

63 

1.96 

.891 

4 

.070 

.070 

34 

.675 

.559 

64 

2.05 

.899 

5 

.087 

.087 

35 

.700 

.574 

65 

2.14 

.906 

6 

.105 

.105 

36 

.727 

.588 

66 

2.25 

.914 

7 

.123 

.122 

37 

.754 

.602 

67 

2.36 

.921 

8 

.141 

.139 

38 

.781 

.616 

68 

2.48 

.927 

9 

.158 

.156 

39 

.810 

.629 

69 

2.61 

.934 

10 

.176 

.174 

40 

.839 

.643 

70 

2.75 

.940 

11 

.194 

.191 

41 

.869 

.656 

71 

2.90 

.946 

12 

.213 

.208 

42 

.900 

.669 

72 

3.08 

.951 

13 

.231 

.225 

43 

.933 

.682 

73 

3.27 

.956 

14 

.249 

.242 

44 

.966 

.695 

74 

3.49 

.961 

15 

.268 

.259 

45 

1.000 

.707 

75 

3.73 

.966 

16 

.287 

.276 

46 

1.036 

.719 

76 

4.01 

.970 

17 

.306 

.292 

47 

1.070 

.731 

77 

4.33 

.974 

18 

.325 

.309 

48 

1.110 

.743 

78 

4.70 

.978 

19 

.344  ' 

.326 

49 

.150 

.755 

79 

5.14 

.982 

20 

.364 

.342 

50 

.190 

.766 

80 

5.67 

.985 

21 

.384 

.358 

51 

.230 

.777 

81 

6.31 

.988 

22 

.404 

.374 

52 

.280 

.788 

82 

7.12 

.990 

23 

.424 

.390 

53 

.330 

.799 

83 

8.14 

.993 

24 

.445 

.407 

54 

.380 

.809 

84 

9.51 

.995 

2-5 

.466 

.423 

55 

.430 

.819 

85 

11.43 

.996 

26 

.488 

.438 

56 

.480 

.829 

86 

14.30 

.998 

27 

.510 

.454 

57 

.540 

.839 

87 

19.08 

.999 

28 

.532 

.469 

58 

.600 

.848 

88 

28.64 

.999 

29 

.554 

.485 

59 

.660 

.857 

89 

57.29 

1.000 

30 

.577 

.500 

60 

1.730 

.866 

90 

Infinite. 

1.000 

INDEX. 


EX                    MAGNETISM.               PAGE 

EX. 

1.  Magnetization      

2 

28. 

2.  Magnetic   attraction   without 

pnntaot 

4 

3.  Locating  the  poles  of  a  magnet 

4 

29. 

4.  Naming  the  poles  of  a  magnet 

6 

5.  Construction   and    use    of    a 

30, 

magnetoscope  

6 

33. 

6.  Effect  of  magnetic  poles  on 

34. 

each  other   

6 

35. 

7.  Effect  of  breaking  a  magnet  . 

8 

36. 

8.  Arrangement  of  poles  in-mag- 

37. 

netization    

10 

38. 

9.  Reversal  of  polarity      .     .     . 

10 

10.  Induced  magnets      .... 

10 

39. 

11.  Magnetic  fields    

12 

40. 

MEASURING. 

A  1 

12.  Measuring  lengths  and  plane 

41. 
42. 

surfaces                           • 

14 

13.  Measuring  volumes  .... 

14 

14.  Using  micrometer  calipers 

14 

43. 

15.  Weighing    

14 

44. 

PROPERTIES   OP  MATTER. 

45. 

16.  Impenetrability   

16 

17.  Porosity      

16 

46. 

18.  Ductility     

16 

19.  Inertia   

16 

47. 

20    Elasticity  of  air 

18 

40 

21.  Elasticity  of  rubber;  of 

4:0. 

stretching  wires    .... 

18 

22.  Testing  the  stiffness  of  wooden 

rods    

2:] 

49. 

23.  Elasticity  of  torsion  of  wooden 

50. 

rods 

24 

24.  Tenacity  of  wires     .... 

20 

51. 

25,  26»  Cohesion  and  adhesion    . 

28 

52. 

27.  Capillary  action  

30 

53. 

PENDULUMS. 

28.  Pendulums 


PAGE 
.     34 


MECHANICS. 

Parallel    forces  in  the  same 

plane 38 

31,  32.   Levers    ....    40-44 

Wheel  and  axle 46 

Pulleys 46 

Inclined  plane 48 

Friction      .......  50 

Center  of  gravity      ....  52 

Influence  of  the  weight  of  the 

lever 52 

Resultant  of  angular  forces    .  52 

Laws  of  falling  bodies  ...  54 

HYDROSTATICS . 

41.  Liquid  pressure 58 

42.  Archimedes'  principle  ...  68 

SPECIFIC  GRAVITY. 

43.  Of  a  rectangular  solid  ...  62 

44.  Of   an  irregular  solid  denser 

than  water 62 

45.  Of  an  irregular  solid  less  dense 

than  water 62 

Of  a  liquid  by  specific  gravity 

bottle 62 

47.  Of  aliquid  by  balanced  columns  64 

48.  Of  a  liquid  by  simple  hydro- 

meter        64 

PNEUMATICS. 

49.  Specific  gravity  of  air  .     .     .  66 
Determine   the   value   of   air 

pressure 66 

51.  Lifting  pump,  force  pump      .  66 

52.  Siphon 68 

53.  Mariotte's  law 68 


164 


INDEX. 


EX.  HEAT.  PAGE 

54.  Heat  from  mechanical  energy  70 

55.  Expansion  of  air 70 

56.  Expansion  of  water ....  70 

57.  Coefficient  of  linear  expansion  72 

58.  Conduction  of  heat  ....  74 

59.  Conductivity  of  water  ...  74 

60.  Convection  in  water     ...  76 

61.  Convection  in  air      ....  76 

62.  Freezing-point  of  water     .     .  76 

63.  Boiling-point  of  water  ...  78 

64.  Effect  of  pressure  on  boiling- 

point  78 

65.  Boiling-points    of    ether  and 

turpentine 78 

66.  Cooling  processes     ....  80 

67.  Dew  point 80 

68.  Law  of  cooling 82 

69.  Latent  heat  of  water     ...  84 

70.  Latent  heat  of  steam     ...  84 

71.  Specific  heat  of  shot     ...  86 

ELECTRICAL   ENERGY   FROM 
FRICTION. 

72.  Development  of  two  kinds  of 

electrification  .....  88 

73.  The  electroscope,  its  construc- 

tion and  use 88 

74.  Charging  by  induction ...  90 

75.  Conductors  and  insulators      .  90 

76.  Potential 90 

77.  Resistance 92 

78.  Electro-motive  force     ...  94 

79.  Condensers,  Leyden  jar     .     .  91 

CURRENT  ELECTRICAL 
ENERGY. 

80.  A  galvanic  cell 98 

-81.  Names  of  plates  and  electrodes  98 

82.  A  two-fluid  cell 100 

83.  A  gravity  cell 102 

84.  Effects  of  an  electrical  cur- 

rent      102 

85.  Electro-magnets 104 

86.  Ampere's  law 106 

87.  The  essentials  of  a  galvano- 

meter .  106 


EX.  PAGE 

88.  A  tangent  galvanometer  .     .  108 

89.  Tests  for  conductivity  of  elec- 

trical currents     .     .     .     .110 

90.  Effects  of  a  neighboring  mag- 

net  on   galvanometer  de- 
flection   112 

91.  Measuring  resistances  by  sub- 

stitution      112 

92.  Measuring    resistance    of    a 

battery  cell 114 

93.  Resistance  by  iVheatstone's 

bridge 116 

94.  Induced  currents ;  idea  of  the 

dynamo 118 

95.  Development  of  the  idea  of 

the  electro-motor     .     .     .  120 

SOUND. 

96.  Vibrations  in  a  rubber  cord .  124 

97.  Vibrations    of    a    sounding 

body 126 

98.  Reinforcement,   interference 

and  velocity  of  sound  .     .  126 

99.  Velocity  of  sound  .     .     .     .128 

100.  Determining  pitch  .     .     .     .128 

101.  Velocity  of  sound  in  a  brass 

rod 130 

102.  Laws  of  vibrating  strings      .  130 

103.  Harmonics  and  beats  .     .     .132 

104.  Interference  of  sound  .     .     .134 

LIGHT. 

105.  Shadows 138 

106.  Law  of  intensity     .     .     .     .138 

107.  Photometry 140 

108.  Images  through  apertures     .   142 

109.  Reflection  from  a  plane  mir- 

ror     142 

1 10.  Reflection  from  spherical  mir- 

rors    144 

111.  Refraction  by  water    .     .     .  144 

112.  Refraction  by  spherical  lenses  146 

113.  Principle  of  the  compound 

microscope  ;    principle    of 
the  compound  telescope    .  146 


A     000  938  029     6 


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JTENORMALSCHOOU 


